My parents gave me white wine mixed with water with Sunday lunch and special occasions from about the age of 7. I personally think it's a very responsible thing to do. The worst thing you can do to a kid is make them think something is banned or dangerous - they're likely to go looking for it.
There was always alcohol in our house - in an unlocked cabinet in the dining room and only once did I open it and help myself to a cocktail of stuff, when I was about 13. I was horribly sick for hours afterwards, and didn't do it again. I don't mean I didn't drink again, I just didn't binge!
Here in the UK the legal age for buying alcohol, or for being in a pub (without an adult) is 18, but there is no law on drinking in your own home.
I think when you look across Europe, where alcohol is a part of daily life for everyone, and children aren't made to think that it is something bad or dangerous, it's clear that that is the way to go.
In most European cities, on a Friday and Saturday night, there aren't huge groups of youths out drinking and drinking with the sole purpose of getting revoltingly drunk. They already know how to handle alcohol responsibly because they've been around it all their lives, and the older members of their families have shown that it is just something to be enjoyed with a meal etc.