It's possible to drink in airports and even buy drinks aboard a flight. Though getting drunk is discouraged, liquor is very much accessible. In fact, you might not even have to buy it. If your flight isn't completely full and you are friendly to the staff, you might be treated to a free drink. You can't expect it to happen frequently because it is hard to comp someone without others noticing and also wanting free stuff.
Next time you fly, try chatting up the flight attendants on board. Being polite and genuine is always appreciated. And who knows, maybe you hit it off and get some free drinks.