Im one that thinks it should be the same age for everything. (outside driving, as thats not really about being adult)
Whatever age you choose, 18,19,20,21, whatever, being a legal adult should mean you are an adult and qualify for everything adulthood comes with. Drinking, Military service, smoking, Voting, whatever.
How can we say on one hand a person's brain is not developed enough to drink but in the same breath say it is enough to serve, fight and die for your country. How can we say someone is developed and knowledgeable enough to vote, but not enough to smoke? It is insanity.
If you are an adult at whatever age you are an adult. I would be ok with the powers deciding to come up with a happy medium and picking an age in between, say 20 for everything, but then making that age the everything adult age.