I think I now understand you. The state may set the Age of Majority, but it would only be a symbolic gesture to be used by private citizens as they may see fit.
For example, the state may set the age of majority for buying drugs at 21 but it would be legal to sell to those under 21?
Or would those parents who felt that was wrong be able to sue if the vendors sold to their minors based on the state sponsored age of majority determination?
You have a child. That child does not have full usage of their intellectual faculties, nor enough life experience to make certain judgements and fully utitilize their Rights. You as the parent are holding those "Rights" in trust until such time as the child is considered an adult and able to use those Rights for themselves.
If your State sets that Age of Majority at 21, then that child does not have full usage of those Rights, on their own without YOUR permission, until such an age. Therefore, YOU could buy your child a beer at a ballgame, but the child itself could not. Any resultant behavior or damage to others from your having your child running around drunk falls directly on your shoulders FIRST.
Is that clearer now?