Let’s just keep it simple.
You aren’t an “adult” until you are 21.
That means everything - drinking, gambling, contracts, gun ownership, conscription, voting, etc. - starts at 21.
Frankly, I’ve had to spend a lot of time with 18 to 21 year olds lately. Most of them have never held down a job, not even a part time one. They don’t know how to file their taxes. They have immature attitudes towards relationships and responsibility.
With our life expectancy getting longer (until very recently, but I digress) it only makes sense to have the age of majority pushed back a bit. Heck, kids are on their parents health insurance until they are 27!
I’m good with it.
Pay attention to that term: Grow up. It's a process. It doesn't happen overnight. People grow into responsibility and maturity by EXERCISING responsibility and maturity. They need to be given responsibility (in small things) as young as possible so that they WILL grow up.
You propose to infantilize 20 year olds??? You're part of the problem.