Is there a set of laws which govern international flights? A few years ago, my brother and I were aboard an Emirates flight from Australia to New Zealand. At the time, I was 16 and he was 14. 

At one point in the flight we were both offered alcoholic drinks. Considering the drinking age where I am from is 18, and ID is required if you look under 25, this came as quite a surprise to the both of us.

Considering different countries have different laws and aircraft often have multinational crew and passengers, what laws govern international flights?
International Airline is bound to follow the law of country of registration. Hence your flight limits you to dringking law in Abu Dhabi which is 18

Note some airline, Turkish here, have some policy not to serve alcohol on certain flights. On my experience with Hong Kong airline, the flight in-bound to Hong Kong only serve juices where out-bound had Tsing Tao :)

A not a really related topic but something more to read on travel.SE : Is it legal to drink your own alcohol onboard a plane over international waters?