I think mostly, as long as kids aren’t being disruptive or drinking in public, that cops really don’t give a crap.
Is it legal in the US for 18-20 year olds to drink alcohol provided by others in private residences?
Yup, service is at the discretion of the establishment, and I believe it also applies to letting 18+ but under 21 into bars. (That’s why I said “can” and not “must.”)
Tsk, come to Milwaukee–I’ll get you some **tasty **beer. 
This is true. I’d always expected this to happen at least once when giving my boys wine with dinner while dining out, but it never did. I was able to pour or order them a glass, no problem, starting around age 14 or so. But I’d been offering them wine with dinner at home since they were much younger just as my parents had, they just hadn’t acquired a taste for anything other than sweet dessert wines then.
I teach in Culinary school in Florida where all students take a Wine and Beverage class that involves tastings. Students under 21 are required to taste and then spit it out. I am not kidding.
Minor nitpick: here in Ontario, the drinking age (at least for buying it) is 19. I think it probably is 18 more or less everywhere else in Canada.
My parents would buy for me and let me drink with them when I turned 18. I prefer beer now, because for me, it enforces moderation. Liquor tastes way too good, and coming home at four in the morning after my staff Christmas party to my 30 weeks pregnant wife(our spouses weren’t invited because my boss is too cheap to pay for them)…not fun
And having researched further here, the legal age is 19 everywhere in Canada except for Alberta, Manitoba, and Quebec.