Then your answer is yes. you support the implementation of a government controlled legal age of majority whereby certain citizens, based on an arbitrary age, are denied by the government from engaging in activites that are legal for a certain class of citizens only because they have reached that arbitrary age.
However you did expand that it would be better to have a 'testable metric' for adulthood but that would be a whole other discussion so I won't diverge at this time.
Also note WHO would be responsible for setting such a metric, the State not the Feds