There is even a scientific basis for this, that the human body, especially the brain, is only fully developed between the ages of 19 and 22 (it varies from person to person). This is a fact.
But how much does alcohol consumption affect this (brain development)? That is debatable.
Regardless of age, it affects reflexes, the ability to think, increases susceptibility, etc.
Regarding driver's licenses and weapons, it really doesn't make any sense. There are American states that have a tradition where children get their first weapons at the age of 11 (usually a small-caliber hunting shotgun).