The second is probably why they people here are surprised the drinking age is 21 in much of the United States. 18, rather than the more traditional 21 (see Roman laws) likely exists as a Schelling point for legal rights, because during the 20th century men of that age where judged useful for military service, not because it was determined as the age where people generally become capable of making all decisions on their own behalf.

Hitler started drafting people of age 22 when he reintroduced the draft in Germany. Later he drafted even people under 18.

I would rather think 18 it's the time where most people leave high school or the local equivalent and go to college or take a job. Taking a job means that you have to be able to make contracts while people act school don't have to make their own contracts.