> Parents don't have any choice in this either.

That's a very fucked up thing to say, governments or random strangers from internet do not have a right to decide how parent raises his child. Do you even have a child?

> I like that you shifted "adults with 15 year olds" to 20 year olds with 16-17 year olds

It was just my own experience, if you want another example in early years of Soviet Union there were 40 year olds learning to read with 6 year olds.

And in general i don't see why any combination of ages should be a problem?