I think at first blush it seems pretty obvious that it's age discrimination to say that those over 18 are in, and those under 18 are out.
But the real point and question—which is why it should give us all pause and why additional study is a good idea—is are the young especially vulnerable? It makes trying to figure out what the right balance is more complicated because the young are more vulnerable. That's the issue you have to wrestle with, so I think caution is in order.