I don't think the setup for the resolution as you propose would yield a worthwhile debate.
It should be illegal to ground people over the age of 18/21
The issue isn't legality or illegality, but whether grounding someone over the age of 18 or 21 should be done in the first instance. When you're talking about issues of legality, that implicates considerations far beyond the more basic question of whether something ought to happen or ought not to happen.
As well, the issue of whether something ought or ought not to happen is one of individual or familial choice; rather than one of societal import (as framing from the perspective of legality or illegality would implicate).