When you register for the draft at 18 yrs of age ,you should be legally allowed to drink and gamble, or make the draft age 21.
Registering for the draft isn't enough. If you get called to duty then yes, lower the age for adulthood for those that serve. But simply registering isn't doing anything special warranting special treatment. Looking around, I would more likely be in favor of raising the age of adulthood from 21 to 25 rather than lowering it.