A) the legal drinking age, age of consent, voting age, and age of Draft/ military eligibility should be the same. Whether that's 21 or 18 or whatever is up for debate, but if you're old enough to kill people at your country's behest you should be old enough to decide what goes in your body.
B) Lots of countries are pretty lax about kids drinking small amounts of alcohol, and it doesn't lead to increased negative outcomes.
I remember getting out of bed, feeling my lower back was stiff, putting my hand on my hip and kind of stretching with a little groan, and realizing that was something I'd seen my dad do multiple times when I was little.