Many of them do. It seems that you are more curious about why not all of them do. Well, for starters, girls don’t get married when they are 14 to 16 like they did in colonial America. Now, because of increased educational demands most Americans don’t get married until their mid to late 20’s. Also, around the 1950’s farms started adding growth hormones to animal feed, speeding the sexual growth of the animals. Since the early 1950’s the average age of puberty has dropped from 16 to 12. So while girls in colonial America sometimes married 2 years before they were biologically ready for sex, today’s teenagers are expected to wait a good 10 years or more after starting menstration. Clearly these are not equivolent in levels of difficulty.