I voted 18, mostly because that's the UK's legal age for purchaisng alcohol, and I think it works out fine. The legal drinking age in the UK is actually age 5 with parental consent; at any age past this, parents can give their children a taste of alcohol, within sensible limits (i.e. a child can have a sip of wine, not a huge bottle). Postponing alcohol consumption to age 21 seems to have no benefit in terms of reducing accident fatalities, and simply serves to make alcohol more of a forbidden fruit, which is absolutely NOT a good thing.
Kids who've been able to try alcohol at young age don't feel the need to guzle it down every time they have the oppourtunity. My parents bought me wine and beer from the age of 16, and I was allowed to consume it as I wished, as long as I stayed under the reccomended limits for consumption. I never got blind drunk at parties because I knew how to handle my drinking. Kids who have no experience with drinking alcohol don't have that knowledge, so when they do get illicit access to do, things go wrong.
We need a complete overhaul of our attitudes towards drinking. It needs to be demystified for kids, and reasonably avaliable to adolescents, so they know it's not a forbidden fruit, or some holy grail of adulthood, it's just another substance that they need to be careful with.