We commonly share and accept the misconception that young people in Shakespeare's times married early. Shakespeare himself has helped perpetuate this misconception with the often quoted lines where Paris observes that women younger than Juliet "are happy mothers made." Capulet counters with "And too soon marr'd are those so early made." In other places, Capulet cautions that his daughter is "yet a stranger to the world" and that she should wait at least two years before marrying. Elizabethan audiences would have agreed with Capulet.
Actually, in Shakespeare's time the average age of marriage for women was 25 to 26 for women and 27 to 28 for men. I learned this while working on a graduate degree in Shakespeare in a class on the social history of early modern England. For anyone interested in exploring customs in Shakespeare's time, I'd recommend our main text for the class: David Cressy's Birth, Marriage, and Death: Ritual, Religion, and the Life-Cycle in Tudor and Stuart England.
On another note, the rash decisions by Juliet and Romeo to seek solutions in suicide underline their inexperience and lack of guidance. The adults who might have counseled them have deserted them or otherwise left them to their own devices. Friar Lawrence considered his own reputation before he thought to stay with Juliet and make sure she didn't do anything stupid.