To assume that once you've passed 18 you suddenly know more about sex than at age 17 is rediculous. everyone develops at a differnt rate, mentally and sexually, especially girls, and to set ANY sort of limit is faulty. I personally think anything after puberty starts is fair game. goes this means they're ready? maybe not. but they know enough by then to say yes or no. it's not it's men forcing themselves upon pubecent girls and claiming child brides.
Environment is also very important. why are some ages of concent lower in some countries but higher in others? It's because in general, the environment in which these kids develope are more open about sexuality than others. Again, a 16 y/o girl in a country where the age of consent is 14 will not always know better than a 13 year old girl in a country where the age of consent is 14.
How are we to judge whether or not any one at any age 'knows' what sex is? There are many adults who can't wrap their heads around child birth, let alone how it happens. i have many friends who lost their virginity in middle school, and knew perfectly well what they were doing. i knew three time as much as my friends knew about sex while i was in elementry school, and that's even after they had their little sex-ed video (thank you TV and an older brother).
Ages of concent and any limit that keeps a pubecent young person from enjoying sex on their own free will is wrong and misguided. It all widdles down to personbal responsibility and education. setting limits only inforces not only a faux sence of morality, but often causes a forbidden fruit effect, having sex for the sake of doing it before the limit line. in a way, ages of concent encourage irrisponsible sexual behavior. cause we all know how well prohibition turned out.