Okay, but the age of consent is 21. Want to change your sex, 21. Take "hormone therapy", 21. Be charged as an adult, 21. Have a driver's license (cars being potentially more dangerous than guns), 21. Buy booze, tobacco, POT, 21 etc. etc. etc.
Research indicates... sorry don't trust them. The research indicates what they want it to, no matter how they have to fudge the numbers. Science isn't cowpies, but scientist/researchers sure do shovel a lot of it.
There is no magic number. I know more than a few 50-year-olds that are not as mature/adult as some 15YO that I know.
I do agree though. You are a legal adult at 18 or 21 stop the sliding scale B.S. Legal being the key word not adult in fact.