The fact that we are sexually maturing at 13 or so doesn't mean anything else is maturing. In reality, we are sexual beings from infancy.
As the parent of two very responsible 13 year olds, I can say without a doubt they are NOT ready for any type of sexual conduct, nor are any of their peers. I have also worked for 3 years with this age group and while some boys looked like they were in their 20's at 14 and the girls had killer bodies, they had the brains of children.
There was a sex educator a few years back making the talk show rounds-Suzy Somethingorother and she had one mantra that really holds true: If you don't know what every part of your body does and can't discuss it openly with your partner, you are NOT ready for sex.
There's a lot more to sexual maturity than pubic hair and breasts. The frontal lobes, the last part of the brain to fully mature and the part that holds our civilities and reasoning, are not 'matured' until close to 20 years old. Hormones don't level out until about the same time.
So while you're looking at that gorgeous 15 year old and thinking she'd be a great catch, she's still a kid, maturing and learning. And, she just might have a dad with a shotgun