"There is a huge variation in skill level in most subjects among eighteen-year-olds in the United States and most places."
I agree with you there, but I do think in terms of ability (if not skill level) then there is an evening out that takes place over time. (I would say more like 25 rather than 18 though.) On a biological level things like developmental delays tend to wash out over time. Even kids with mild autism tend to become more or less normal after a while. And on a social level people become less dependent on their personalities, which is important since a lot of what is considered ability is actually probably just personality to begin with.
I agree with you there, but I do think in terms of ability (if not skill level) then there is an evening out that takes place over time. (I would say more like 25 rather than 18 though.) On a biological level things like developmental delays tend to wash out over time. Even kids with mild autism tend to become more or less normal after a while. And on a social level people become less dependent on their personalities, which is important since a lot of what is considered ability is actually probably just personality to begin with.