We do not have last names for Benvolio or Mercutio. You might note in an obituary that Benvolio is a nephew of Lord Montague and that he is Romeo's cousin, and that Mercutio is related to Escalus, the prince of Verona. Mercutio is also a close friend of Romeo's.

We know that Juliet in the play is almost 14, but other ages are not provided. Romeo is almost certainly a teenager, given his impulsiveness, lack of maturity and the fact his parents worry about him the way parents generally worry about a teenager: as the play opens, his father, for instance, can't understand why Romeo is mooning around his room all day. Given that Romeo is most likely a teenager, we would expect both Benvolio and Mercutio to be teenagers too. Benvolio's solution to Romeo's lovesickness is to advise him that there are plenty of pretty girls in the universe and to go out and find one (fairly standard teenager advice to the lovelorn). Mercutio has the energy, high spirits and lack of forethought we associate with the teen years, and his probable youth makes his death all the more tragic.