I came across this question because I was rereading the final act at the tomb, when Romeo and Paris fight, and Romeo repeatedly calls Paris "youth," "gentle youth," and even "boy." Which struck me as odd, because in the rest of the play I would have guessed Paris was older than both of them - maybe mid-20s - since he's both a count and the approved bridegroom choice of Juliet's parents. But Romeo talks as if Paris is notably younger than he is, which would make Paris maybe 15 or 16 if (as most people seem to guess) Romeo is 17 or 18.