You can drive a car at 15-16 most states, which puts other people's lives in your hands. You can get married between 16 and 18 depending on the state. You can join the army, take on debt, and do pornography at 18, all three of which will completely alter your life long term. You can drink, buy a gun, or enter a casino at 21. These feel someone nonsensically ordered to me. Seems like we should just pick one age for "you can now make and be responsible for your decisions" and stop talking about all of these weird edge cases as if we need the law to deal with all of them.