OK, so I realize this is just anecdotal, but I would like to share my “story” anyway: I grew up in a family which regularly saw my parents drink alcohol, and in a family which served me and my six siblings alcohol from a young age. When we were younger, it was usually just a little wine with Sunday dinner, and my dad would usually water it down. As we grew older, usually once we were teenagers, we were allowed to have small amounts of all kinds of alcohol, not watered down, and definitely later on in high school, the amount we were allowed increased. Of course, with seven kids, by the time the youngest was later on in high school, my dad was allowing him to have a full beer or something here and then. In no way was this harmful to us, our peers, or anyone our family hung out with. It was always at home, and my parents, if we were at a trusted friend’s house (so still private property), at an appropriate age (like, high school) would have given consent to consume alcohol (although such situations rarely happened).
We are all now grown up. My youngest sibling will turn 21 within a couple weeks. None of us are alcohol addicts; none of us go out and get tipsy all the time and especially not drunk (although with a few of us, it has happened before, I admit). When we drink now, it is usually an alcoholic beverage with dinner or just enjoying one in the evening, or a few drinks (within reason) in social settings/gatherings at home (or even occasionally in public, since we are all of age now).
Needless to say, generally I agree with the OP in that the parents are the best ones to make the decision on private property. That being said, I can understand how one can look at the statistics in America regarding binge drinking/underage drinking, and can say that the government is legitimately setting rules against underage drinking with parents’ consent on private property. In the right family setting, though, it is clear there is nothing amiss with parents letting their kids drink from a young age, even if it is against the law. To add to my “anecdote”, all of my siblings and I are law-abiding citizens and are not overly “casual” with the law (aside from the obvious things right now regarding abortion and now same-sex marriage).
I guess my only point in relating to you my story is that me and my family (and many of my family’s friends) are living proof that there’s absolutely nothing wrong with young kids drinking, and that it will not inevitably lead to binge drinking and whatnot. I will admit that we all drank more in college than before or after, and that there were occasions on which they drank too much, and I don’t want to excuse that behavior, but it really lessens the gravity of it when you are only with friends and not like, going out driving or something afterwards, only going to bed at home. And I suppose some will use that against me to say, well, that must be a result of them drinking from a young age… fine, go ahead. But I think being away from parents, and going out and drinking with friends at college is a little different situation. Again, my main thing was that I think there’s really nothing wrong with parents providing alcohol to their children on private property and according to their age, providing they have an adequate understanding of the appropriate usage of alcohol.
Obviously it’s great to be able to have fun without alcohol, but there’s nothing wrong with drinking alcohol in moderation with the intent to simply “be joyful” in your fellowship with family and friends. For me and my family, it added more joy to the joy which was usually already present!

Drinking is a very “Catholic” thing to do, and I wish more people could be exposed to appropriate drinking from a young age, or at least that from a young age they could be exposed to the idea that alcohol is something that can/should be legitimately enjoyed. I suppose that every family is different; some families I think are exposed to that idea but still never drink, and I guess that’s fine too. Again, for my family it’s really just engrained and I will never be convinced that drinking from a young age is inherently bad for the kids - rather, it depends on the situation, and in many situations it is good.