A 20 year old is an adult, not a "kid"
At 20, you can't drink in the US. I don't think you can buy nicotine, and probably can't buy weed in legal places.
Do they still deny rental cars to 20-year-olds, either outright or through fees too high for most 20-year-olds? Is this still even if their car insurance says they get a rental? Do some hotels still deny young folks lodging?
Sexual development, ability to bear children? Mid teens for most, earlier for some.
Completed puberty, full height? Mid to late teens.
Finished the fastest phase of brain development? Mid-20s.
Even if you want a biological standard, there's still plenty of subjectivity and many of these markers are quite a bit earlier or later than people would be comfortable with.
Given that, I think it's fine to focus instead on legal or social aspects of adulthood. And full legal adulthood in the US has been 21 for some time and it seems to me that like my country, the social definition of adulthood is the end of full time education (including in my lifetime undergraduate education being considered an extension of childhood education)
>Sexual development, ability to bear children? Mid teens for most, earlier for some.
Let's say 15
>Completed puberty, full height? Mid to late teens.
Let's say 18
>Finished the fastest phase of brain development? Mid-20s.
Let's say 25
The average of 15, 18, and 25 is 19.3
Couldn’t drink (outside Louisiana) and legal weed wasn’t even a thing.
Hell, I was jumping out of airplanes in the army when I was 17.
Seems relatively “adult” to me.
Adult is realistically mix of how society treats you and your development.