As you might expect I did a fair share of my underage drinking at that bar.
Now, I was born in 1970 so by the time I got to high school the legal drinking age was 21.
You see how that works?
Drinking was limited to folks 21 years of age and older, but since nobody was looking we regularly took advantage of the fact and drank, despite what the law said.
There's an old saying that "locks are made to keep honest people out".
The same can probably be said for unenforced laws.
Without actually establishing by some simple, rudimentary process to verify that folks showing up at the polling place are actually legally entitled to vote, or kids showing up at the bar are actually legally entitled to drink, how else would we enforce the laws that underly the legitamacy of either action?