The truth is there is no good age. Some people have their minds right for it at 14 and some don't until 25.
Problem is we're in a weird cultural situation where kids are grossly "sexed up" physically while simultaneously infantilized emotionally and mentally for literally their whole lives.
Start looking up your ancestors and you'll see a lot of marriages between mid-late teen girls and dudes in their late twenties/early thirties. It worked then because you couldn't fuck around (no birth control pills), they had strong social circles to chaperone, and kids had to deal with shit like Spanish Flu and parents that were actually around, so they were mature enough by their teens to have a grasp of the responsibility that's inherently tied into sex.
Anyway, a case-by-case basis is the only good solution. The important thing is differentiating between a twentysomething guy banging a post-pubescent teen and someone diddling a ten-year-old. Bodies mature at different rates, brains mature at different rates. I think 16 is a safer age just to err on the side of caution for teens that look older (I've known of guys and girl that have committed felonies because they didn't ID before sex), but the cutoff could be as high as 20 with parental consent required under that. Of course, all this is tied into all the other arbitrary ages for things like voting, tobacco, alcohol, driver's licenses, competency to enter into contracts, etc.