because obviously you don't wake up on your 18th birthday magically a completely different and more intelligent and mature person than you were the night before. But I can't really think of a better way to do it. Take the example of drinking... obviously you can't let an 8 year old go in and buy a bottle of gin... they could drink the whole thing and die of alcohol poisioning because an 8 year old is not intelligent enough to drink responsibly. So we need some kind of restriction. Making that restriction 18 or 21 is arbitrary, yes, but how else do you do it? Judging on a case by case basis is fairly unfeasible, because how do you objectively judge whether somebody is mature and responsible enough to drink responsibly?
Now, I wouldn't be opposed to something like between the ages of say 14-18 you can do things like drink, drive, have sex etc. with your parents permission, because if anybody can tell if you are responsible enough, it would be your parents. But there still has to be some kind of age where you can legally decide for yourself, because otherwise we'd have perfectly mature and responsible 25 year olds with very strict parents still not giving them permission to do things.
Now, I wouldn't be opposed to something like between the ages of say 14-18 you can do things like drink, drive, have sex etc. with your parents permission, because if anybody can tell if you are responsible enough, it would be your parents. But there still has to be some kind of age where you can legally decide for yourself, because otherwise we'd have perfectly mature and responsible 25 year olds with very strict parents still not giving them permission to do things.