Sorry for the rant but here the load down on the R&J. J is 13 but the book never tells the age of R. However, since the book states that he has a sword we know for a fact that boys get a sword from 14 and up. We also know that he is older then her by a couple of years which put R in the 16-20 age range. Then it the time frame of the the story. This did not happen over the course of three days, it probably happen over several months. Why it seems only three day's because people have lives and the play would have taken several months to produce. So think of it this way it is similar to the LTR/H books. The R&J play is as thick as the Hobbit book but the time frame is as long as the LTR trilogy.
But by your logic it would make perfect sense that there are child brides.