People of all ages are involved in the brawl in Act 1, Scene 1. We know this because Capulet asks for his sword, so there are older people there too.
There is no evidence of exactly how old the participants of the street brawl are, but most of them are young men. We can determine this by their rashness, and by the fact that they are servants and Romeo’s friends. We know that Tybalt is Juliet’s cousin, and he is somewhat older. Of course, the parents are also involved, and they are older.
One of the reasons the servants seem young is their bawdy talk. They make jokes like this one.
'Tis true; and therefore women, being the weaker
vessels, are ever thrust to the wall. Therefore I will push(15)
Montague's men from the wall and thrust his maids to
the wall. (Act 1, Scene 1, p. 9)
This is locker room talk. In this case, it does not take much imagination to figure out he is really talking about sex with women.
When the prince scolds them, he calls them “men” and he is annoyed because it is not just the kids who are fighting.
What, ho! you men, you beasts,
That quench the fire of your pernicious rage(80)
With purple fountains issuing from your veins! (Act 1, Scene 1, p. 13)
Clearly the feud has gotten to the point where two servants can’t walk by each other in the marketplace without starting a huge fight in which everyone nearby takes part.