Well, it used to be 21.
OTOH, when I was in the Air Force I was responsible for repairing multi-million dollar aircraft that other mens' lives depended on working properly.
So I think its a 'nature vs. nurture' thing. If you raise a SoyBoy, he won't be an adult until his 30s. If you raise a man, he could be a man at 17.
I think you have a good point. You get a responsible mature man by giving him progressively greater responsibility, and demanding at each stage that he act with judgement and maturity.
There’s also a reason why the military puts 18 year olds under the close supervision of much older sergeants.