Here's a chunk of a wikipeda article.
On average, girls begin puberty at ages 10-11; boys at ages 11-12.[1][2][3] Girls usually complete puberty by ages 15–17,[3][4][5] while boys usually complete puberty by ages 16–17.[3][4][6] The major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12-13;[7][8][9][10] for males, it is the first ejaculation, which occurs on average at age 13.[11]
I've done some light research and it appears to be fairly accurate, I did find a parenting site that states the following:
Girls are more likely to experience early puberty than boys are. A major reason for this difference is that girls generally undergo puberty earlier than boys normally. Girls also exhibit a larger range of puberty onset (9-16 years of age versus 13-15 years of age for boys), making earlier onset more likely.
So...surprise! Boys and girls are different again. The thing is that being physically ready for the act of sex doesn't prepare you for it. And the act itself is psychologically different on a very profound and fundamental level for the two genders.