teenage sex??

Paisley
i always hear "teens arent ready for sex" "their bodies cant handle it" and that sex should be put off until older. but where is the facts behind this? i am 15 and am not a virgin, i know lots of teens who are sexually active. if both partners give consent and are excited about performing sexual activities, and are using safe methods and know/understand the risks of teen pregnancy, what is the harm? does sex physically harm teenagers? why do adults and parents tell kids this?