legally speaking, that magical age is also a matter of geographical location.
You could fuck a 16 year old in one place, move 10 feet across an imaginary line and suddenly be a criminal.
Frankly, I think the ability to have sex should be a licensed thing like driving.
You take a test at 13, and if you get a license, as long as your parents approve, you can have sex with anyone. After 18, you don't need parental consent.