13 for girls 16 for boys Lets face it, girls mature much faster than boys. Most girls hit puberty between 9 and 11 and typically fully developed by 13 or 14 (not in all cases but most). Buys on the other hand hit puberty between 12 and 13 and don't typical fully develop until 16 to 20. I'll focus on the girls since that's the one that I think should be changed. As I stated about, girls are fully developed at 13. A girl who looks like she is around 19 or 20 is most likely 13 or 14. I can't tell you how many times I have tried to pick up on a girl thinking she was 20 only to find out she is actually 13. This created a potentially dangerous situation for the guy if the girl decides to lie about her age. There a many younger girl that actually like older guys and if willing to lie about their age to get with one could cost jail time for the guy. No guy is going to ask for a girls ID before having sex with her. The fact is that 13 year old girls know what sex is and some even want it and with who should be totally their decision be it with another teenage boy or someone in their 30's.