"Adulthood" is just some arbitrary number derived from parents who don't want their kid around any more, and kids who don't want to live at home any more
Really, the sex drive starts around 12-13. Age of consent should be determined by 4 factors: that there is a drive and desire (consent), knowledge of safety (including condom use), reality (average age for losing virginity is now like 15), and avoiding manipulative situations (age and status proximity)
There are plenty examples of highly successful people who lost their virginity at 13. If they use condoms and they're peers, sex isn't some deadly albatross. There are perfectly legal activities that are more dangerous, like football, that we allow at even earlier age. There are some that are even more common and dangerous, like drinking, that happen openly. IMO the problem is we often place our entire self worth on it, which both encourages sex before we're ready, and encourages laws that *create* victims where there is none (consensual underage sex)
If there is doubt, an individual assessment of the kid's maturity is more appropriate than a blanket law that treats everyone the same, when they are not