19 is more sensible, that way there aren't any mixed drinker/nondrinker parties in high school.
Yes, for two reasons:
1) Legal or officially sanctioned parties (think the prom or something) would probably have alcohol for the 18-year-olds, which is unfair for the younger types
2) Those who are actually concerned about what the law states (e.g. me, other weirdoes) might go to parties with other people who are legally consuming alcohol at their age, and what would happen then? I mean, parties with kids serving alcohol underage are obviously going to have underage drinking either way, but having some seniors who can drink legally and some seniors or members of other grades who can't would make stuff too fuzzy and unenforcable (even by the hosts themselves, if they want to enforce it)
