Okay, I think I get the lower age limit, since they presumably have illegal parents that should be sent home—though that further works against the idea that it would be somehow cruel to send the kiddos home with them (as if the parents would abandon them here anyway) — but if they are older and have completed school and not been criminals otherwise, I don’t see why they should go home sooner than the demographically young adults should.
Your point is well taken. The upper limit is rather arbitrary. I dislike the whole concept but I think it will happen and I’d like it to have a lower rather than higher age limit, not 30! With kids and stuff. No.