It doesn't matter what the science says.
The reality is you can't keep infantalizing human beings indefinitely b/c we find this is harmful until that age, while that is harmful until this age in an every growing complex set of rules.
Deciding what an "adult" is, yes - is arbitrary. And the closer you get to it (16, 17) certain things become muddy (like how to charge for a crime).
But the line needs to be drawn somewhere, and it needs to be consistent. It's simply a matter of principle. Either a person gets the rights of a full person under the law, or you dont.
Expectations that you pay taxes, that you can serve in the military, that you have all kinds of autonomy but can't enjoy a beer b/c ZOMG.. it could hurt development?
Do you think alcohol somehow becomes a juice cleanse after 21? No - it can cause liver failure, cancer, contribute to heart disease, diabetes. It's not like you become an adult and this is all of a sudden the fountain of fucking youth. It's poison. It will always be poison. When you're 60 years old? It's still poison.
And since i mentioned 60 year olds - if you take your philsophy of "what the science says", then why not just bring back prohibition on all sorts of things? Lets stop marijuana legalization in its tracks. Lets ban smoking. Lets tell people who are increasingly fragile due ot their age what their diets should be. Lets cap drinking at what? 50? 60 years old. I mean, it can't possibly be good for an 80 year old to enjoy a glass of whiskey. So lets ban that too.
Fuck that.
You're either an adult or you're not. End of story.
Prohibitionist excuses grow increasingly complex as years go by, but it doesn't change the fact that it's fucking draconian and antithetical to the freedoms people expect out of their society that they contribute to. It doesn't matter if they're on the police force, the military, or just a tax paying citizen.
paying taxes for a nation you don't have full rights to, is crap. being able to vote, while not even being a full citizen, is crap.
20 years from now, someone's going to be finger waving to raise ages to 30 or 40 b/c some twerp in a lab coat has some study that says this or that.
It doesn't fucking matter.