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)