Do people really believe that 20 year olds are children and no one ever in the entire history of the world ever noticed this, and the only people who ever got this right are in the USA in the last 20 years?
Personally, I think most 20 year olds are mature enough to be called adults.
I don't think a 13 year old is.
I agree with most of Europe that around 18 is a sufficient age.
So for me, 18 is the point where people are matute enough on average.
Your first comment suggests you think as soon as a person hits puberty they're mature enough to be called adults. I'm not sure at this point if this is what you meant.
I have seen people literally argue that a romantic relationship MUST be abusive because one of the lovers is only 19 and therefore "still a teen" ... the dogma that says teens are children is absolutely false and it is damaging the hell out of young people.