general rule-if you look old enough to drink, they'll offer it to you. If they don't offer and you aren't old enough, don't ask.
British Airways got me good when I was 20. Two beers before dinner, a small bottle of wine with it, and 2 more after.
and if you're in first class, odds are you'll be drinking heavily before the economy passengers are even on the plane. I was old enough to drink already, but in business class United was feeding my champagne like none other before we even left the gate.