16, but for goodness sake we need to inject some common sense into statutory rape laws. A 17 year old who has sex with a 15 year old is not a criminal.
The vast majority of people charged with sex crimes are just kids. It's obviously a flawed system. Keep the 16 limit, but don't take it so literally.
In many states there is what is called a "
Romeo and Juliet clause"
Emotional maturity is a tricky thing to gauge. I would think that a 3 year window for those over 14 would probably be a fairly lenient standard. Although 18 is a somewhat arbitrary age for 'maturity' (I think that there is often a massive difference in maturity between 18 and 21 or 22), but it is a societal norm and most people have at least the basic concept of personal responsibility and simple abstract reasoning skills.
When both parties are young and close in age, what they need is guidance and help to reform, not juvenile hall. If one party is significantly older, then it's exploitation and a far more reprehensible crime. (and yes, that includes situations where the older party is female - exploitation is exploitation)