NeutiquamErro: Why do you say that? The laws banning underage participants in any activity isn’t because they don’t have rights as children, but because it is deemed dangerous and they are not considered capable of looking after themselves in this area.
If it could be proven (which is irrelevant to your point, I think) that humans aged 20+ were capable of driving responsibly but were not able to drink responsibly until they were 22, why would you be against a law tailor for maximum safety? To prohibit this 21 year old from driving would be ridiculous, because he is capable, but to allow drinking would be irresponsible and damaging.
Are you against laws that protect people when they are adults? Do you feel all drugs should be legalized to adults, because they are capable of making their own decisions?
I don’t know why you look at ‘adult’ as a boolean attribute. It is entirely relative.