I'll be flying from Sweden to Iceland on Icelandair in a week. Will they serve me alcohol when I'm 18 years old? The drinking age in Sweden is 18, and in Iceland it's 20. If they will serve me, I'll upgrade to Saga class.
Oddly enough I wasn't able to find any information on this online, I'll continue to search in case I'm able to find something.
Meanwhile, I'll tell you the general rules. As you would expect, it is entirely dependent on the policy of the country in which the airline is registered. Considering that, since the drinking age in Iceland is 20, the drinking age on the flight should be 20.
There is a catch here though, but it's not the fact that the flight it going to Sweden. This is not considered in this case and the flight could be going anywhere and the drinking age would still remain the same.
The catch is that most of Europe has a drinking age of 18, so it is possible that in order to maintain compatibility with the rest of Europe, Iceland allows Icelandair to follow a policy wherein the drinking age is a more widely accepted 18.
As far as I would presume though, if you look old enough, they won't specifically ask you to present your ID, but of course I would wait for someone with personal experience to probably verify.