to think about the bigger picture and about strategic alliances between families; the prospective marriage of Paris and Juliet is just that: a strategic alliance between two great families, not a union of hearts.

Ultimately, this gaping chasm between the generations cannot be closed, at least not without something drastic happening to bring about some kind of reconciliation. The something drastic in question is the tragedy of the star-cross'd lovers' deaths, which comes about largely due to a breakdown in communications between the generations.

Youth and age are certainly contrasted in Romeo and Juliet, though not necessarily old age. It is rather a contrast between youths and adults.

Youth is portrayed through the young characters of the play, such as Romeo, Juliet, Benvolio, and Mercutio. Juliet is our youngest character, being only 13. We know that Juliet is only 13 because her father points out to Paris that she is not yet fourteen in saying, "She hath not seen the change of fourteen years" (I.ii.9). Likewise, Juliet's Nurse points out that in a little more than two weeks, Juliet will be 14 (I.iii.18-21). We are never told exactly how old Romeo is, but since he and his friends are men roaming around town, we know that he and his friends are at least in their late teens, possibly early twenties. If they were younger, they would still be at home studying under their tutors, which was the way that boys of high society were educated back then.

All of these young characters make extremely emotionally driven, impetuous, and rash decisions. Even Benvolio, who is known for being the peace maker, comes up with the foolish idea to crash the Capulet's ball. We can assume that crashing the ball was his idea because, as the second speaker in the scene, he is the one to lay out the plan in Scene 4 of Act 1. Both Romeo and Juliet also make the rash, emotional decision to marry so suddenly. The whole play is full of youthful, emotionally driven, rash decisions.

In contrast, some of the characters that represent the older, adult generation of the play are Lords and Ladies Capulet and Montague, Friar Laurence, and Prince Escalus. But even these older characters are not necessarily wiser. They, too, are known for making emotionally driven, rash decisions. Lords Capulet and Montague have made the decision to carry on a feud that was actually begun by their ancestors, as we learn from the line, "From ancient grudge break to new mutiny," found in the opening Prologue (3). Their decision to fight actually goes against their own known better judgement, as we learn from Capulet when he declares "and 'tis not hard, I think, / For men so old as we to keep the peace" (I.ii.2-3). Friar Laurence decides against his own better judgement to marry Romeo and Juliet, simply because he hopes it will end the feud. He also makes the irrational decision to fake Juliet's death, rather than to announce her marriage to her father. Even Prince Escalus, who is known as the voice of righteous judgement and pure reason, confesses to making poor decisions. He repents not putting an end to the feud sooner as it cost him Mercutio's life, his own family member.

Hence, we see that while youth is contrasted with the older generation in the play, both the youth and the adults actually make the same foolish, emotionally driven, impetuous, and rash decisions, showing us that so long as you allow yourself to be governed by your emotions, there is actually not a vast difference between youth and age.

Teenagers are represented as passionate and strong-willed, but also compulsive.  They are also represented as victims of adults.

It is easy to suggest that Romeo and Juliet are a couple of silly kids who got mixed up in things they did not understand and paid for it with their lives.  However, if we look at things more closely we notice that it was actually the adults that made the most mistakes.

Shakespeare says it best in the initial prologue.
A pair of star-cross'd lovers take their life;
Whose misadventur'd piteous overthrows
Doth, with their death, bury their parents’ strife.(Act 1 prologue, p. 8)

It is the parents’ strife.  Yes, Romeo and Juliet do get into ‘misadventures,’ as all teens do. However, if the parents were not feuding then they would not have had such tragic consequences.

Yes, Romeo and Juliet acted compulsively and selfishly.  They were interested only in themselves, and did not care about their parents’ wishes.  But who can blame them?  Look at what their parents wanted?  They wanted them to hate another family simply because of the name.  They wanted them to fight each other.  They wanted Juliet to marry a man she barely knew to advance her father’s social status.

Romeo and Juliet is a play about mistakes, but it’s important to recognize that the kids are not the only ones making them.  Friar Lawrence fully acknowledged his part in the young couple’s death.  What was he thinking?!  If he had not given Juliet the potion on the long shot that she could overcome Romeo’s banishment and get her parents to see reality, she would not have died because Romeo would not have killed himself thinking she was dead.

Juliet’s father wants to relieve what he thinks are tears for Tybalt and so doesn't understand when Juliet is not proud to be betrothed to Paris.

Soft! take me with you, take me with you, wife.
How? Will she none? Doth she not give us thanks?(145)
Is she not proud? Doth she not count her blest,
Unworthy as she is, that we have wrought
So worthy a gentleman to be her bridegroom? (Act 3, Scene 5, p. 83)

When Juliet and Capulet disagree, Nurse comes to her defense, telling Capulet, “You are to blame.”  In the end, it is his insistence that she marry so soon after the feud has resulted in Tybalt’s death that leads to her destruction.

So while the teenagers are represented as reckless, their parents are shown to be even more so.  It is easy to get caught up in blaming the title characters for being too free with their love, but their parents and other adults are just as much to blame for the tragedy.

This question leaves some room for subjectivity, as cultural attitudes on what young people should be like differ throughout space and across time. However, I will try my best to relate Shakespeare's characters in the context they were written and how we might think of them today.

Romeo and Juliet is set in a time when coming of age occurred much earlier than most consider it today. Juliet, who is just thirteen, is already considered to be of age and eligible for marriage. Bear in mind that while it was not uncommon at the time to betroth a child well before they would actually marry, Juliet is considered to be grown and so her mother is putting a lot of pressure on her to marry Paris. This expectation of Juliet to take on an adult social role speaks to a situation we can observe in other parts of the play-- many of the central characters are young people who seem to have adult roles thrust upon them before they are emotionally mature enough to handle them.

Shakespeare certainly had no problem crafting characters with distinct personalities, and this is evident in Romeo and Juliet. We encounter the dreamer Romeo, peaceful Benvolio, and fiery Tybalt, among others. What all of the young characters have in common is that they are passionate, often acting to fulfill their own emotional desires rather than based on what might be objectively helpful. As terrible as it may sound, I think that Shakespeare wanted to portray young people as capable of deep thought and feeling, but ultimately slaves to their own emotions. Of course, it is not the fault of the young people that they were expected to function in adult roles and responsibilities, but their self-serving natures proved disastrous. 

It's actually not as uncommon as one would think for a man to feel attracted to a young teenage girl. Girls develop much faster than boys, and at the age of 13, or even 12 or 11, girls can look deceptively like young grown women. We can assume that Juliet may have been a fast developer as not only did Romeo fall in love with her, but Count Paris was in love with her long before Romeo even ever saw her. We know that Paris has been in love with Juliet for quite some time as in the very second scene we see him asking Lord Capulet for Juliet's hand in marriage, not for the first time, but for the second time, which we can gather from Capulet's response to Paris's question, "What say you to my suit?":

But saying o'er what I have said before:
My child is yet a stranger in the world
..
Let two more summers wither in their pride
Ere we may think her ripe to be a bride. (I.ii.6-11)

Since the phrase "but saying o'er what I have said before" means that Capulet is repeating himself, we know that Paris has asked for Juliet's hand before and has received the same answer before. Therefore, we know that Paris was in love with Juliet well before Romeo ever saw her and that she must, therefore, be a very fast developer.

However, while marrying at a young age certainly did happen, it may not have actually been as common an occurrence as the play or other works of literature indicate. While Romeo and Juliet was written during the Elizabethan period, the play actually reflects historic events that took place during the Renaissance. During this time period, it was certainly legal for girls to marry at the age of 12 and boys at the age of 14. Many girls did marry close to this age because the life expectancy rate was much lower than it is today ("But Was It Love?"). However, studies through the analysis of marriage records have shown that, especially for the noble class, the average marrying age was in the mid-twenties. Upon average, women married around the age of 23 while men married around the age of 26 ("The age of marriage").

Therefore, while Shakespeare's play depicts a common occurrence, it does not depict the average occurrence. While it is not unlikely for a man to fall in love with a teenage girl, it certainly isn't the average occurrence. For today's audience, the young ages of the couple add to the drama and tragedy of the play; it's very likely, due to the average marrying age, that an Elizabethan audience would have responded in the same way.

To answer this, I think a good place to begin would be to consider characters of various ages and how they are presented in the play. The lords and ladies Capulet and Montague are certainly not presented very favorably. Lord Capulet, for example, threatens to disown his daughter when she refuses to marry Paris (even though Capulet has said earlier that her feelings are important to him). Lady Capulet does nothing to protect her daughter from her husband's rage. Lord Montague is ready to pick up his weapon and join the fray in the beginning of the play. This older generation seems to be presented pretty negatively: they are violent and completely lack concern for anyone who is not within their own family. Friar Lawrence isn't violent or apathetic toward others, but he can be seen as guilty of a terrible error in judgment: marrying these kids without their families' approval and then helping Juliet fake her own death so that she can run away and be with Romeo.

The young people are somewhat more promising. While Tybalt is awful, Benvolio is peaceful and honest. Romeo and Juliet are so much better than their parents; they can look past their families' hatred for one another and follow their hearts. In many cases, the children seem to be the victims of their elders' bad behavior.

While our society considers this to be inappropriate, it is important to keep in mind that Romeo and Juliet was written for a very different audience. In Elizabethan times, a girl could get married at the age of twelve if her parents gave their consent. While this was not a common practice, it was still considered acceptable. 

In addition, members of noble families married earlier than the lower classes. The Montagues and Capulets would both fit in the noble class. This was to build alliances between powerful families, which allowed the audience to accept the reality of a young girl getting married so early. 

One of the most important factors is that Romeo and Juliet is a Shakespearean tragedy. It is meant to make the audience feel, and hyperbole is one of Shakespeare's greatest tools. Having characters die at such a young age would have a more powerful impact than if they married when they were older. In the end, it is the drama of it all that wins over sense.

You could talk about the Nurse and Friar Lawrence.  For the most part, they're the only level-headed characters in the play.  Both are adults, in elder years.  Most other characters act irresponsibly and childishly.  For example, Romeo, Juliet, and their friends (Mercutio, Tybalt, etc.), are presented as hot-headed, quick to move, and easily infatuated.  Friar Lawrence calls attention to this when Romeo goes to him to ask Lawrence to perform his marriage to Juliet.  Friar Lawrence mentions that just the previous day, Romeo had been bemoaning his unrequited love for Rosaline.  He warns against Romeo acting too quickly.  In this way, Lawrence calls attention to Romeo's childish nature, that he is like a child who moves from one toy to another without pause.

The nurse is similar.  Following Romeo's banishment, Romeo mopes in Friar Lawrence's cell.  Even the Nurse tells him to grow up!