I think potential merit is important. Enlisting in the military is a life-changing, possibly life-ending decision. If we say someone is old enough to make the decision to enlist, then they are certainly old enough to make the decision to buy and drink a beer.
Potential to enlist isn't the same as enlisting.
Considering defending the country isn't the same as doing it.
Thinking about service to country isn't the same as serving it.
Let those who actually chose to defend the country enjoy an additional privilege.