The US does have some interesting laws about recreational inhalation or imbibing. FWIW, elsewhere in the western world teenagers can't buy booze, but at restaurants or social occasions with parents they can drink alcohol without being furtive about it. The US 20-year thing is unusually strict.

A similar age limitation for weed can be justified because weed (like alcohol) can have an effect on young brains, as well as the usual risks of smoking.

But the reality is that people who want to drink will drink and people who want to smoke weed, will smoke weed. Police forces in many university towns mostly turn a blind eye because there's no point sweating the small stuff and because they realize that Prohibition empowers criminals and creates exposure to harder drugs.

Legalizing weed is a good way to ensure that the law is not an ass. Smokers already are segregated from non-smokers in public places so those who disapprove, are unlikely to experience much of a difference. Meanwhile the taxmen are rubbing their hands with glee. ;-)