But none of those (that I know of anyway) specify you have to be 18; the exception is some combination of parental consent, private location, medical or religious reasons, etc.
That's the point. You can't purchase alcohol in the US prior to being 21, but there are lots of situations where consumption by someone under 21 is allowed.
I've seen people proposing that allowing children to consume alcohol with their parents permission would help with the binge drinking culture, but that's already legal in 35 states[1]. So I posted my (slightly pedantic) earlier comment out of a belief that this is something that is not widely understood.