I think the whole "you are a adult now-you can die for your country, enter legal contracts, do dangerous work, etc...but ...beer/booze is FAR too dangerous! none for you for 3 more years!"
Make ALL rights at one age-none of this "Partial" adult stuff. If you don't want people to drink untill 21, adult status should be give at that age.
Make adult age 19 or 20 and give ALL rights at that point. YOU are responsible for yourself at that point. You get a DUI and cause an accident, YOU pay the price. Ask Canada about that-DUI is a felony there. When I visited Victoria, BC last year, a friend and I were going out to drink/hang out with friends..his first words were "suck it up and take the bus there, taxi back if need be. Believe me, you DO NOT want to get a DUI here..it is a felony and the punishment is pretty bad".