These are good questions, in part because there are no good answers. It isn't clear that 18 is a better age of majority than, eg., 17 or 19. And there are huge inconsistencies between minimum age policies for different activities.
For example, where I live, you can get a drivers license at age 16, join the military at 17, apply to become a police officer at 18, get married at 18 (or 16, with parental permission). But you can't buy alcohol until age 21.
Some other states, the minimum age for drivers license is 14, the minimum age of marriage (with parental consent) is as low as 12 or 13 (some states even have exemptions that apply below those ages), and there is no minimum age to become governor! Thus, middle school students have run for governor in Kansas and Vermont, in recent years.
Yet, every state has 21 as the drinking age, in large part because the federal government will deny part of their highway grants if they don't. But, 30 of those states DO allow 18 year olds to become bartenders, and several more allow 19 year olds. Thus, an 18 year old is mature enough to sell and serve alcohol, but NOT mature enough to buy it himself (even though a bartender has obvious opportunities to drink alcoholic beverages to excess, if he/she wants to).
It doesn't make any sense.
