On an Alaska Air flight 5 years ago, I was flying with my 12 year old son. They had mistakenly put us in an exit row (only seats together). When they found out, they moved him but not me (too hard to find passengers willing to move, I guess).

Last month the same son (now 17) and I flew to Maine, with exit row seats most of the way both directions. Every time my son was questioned about age, and twice, even though he is not young looking, was asked to show proof of age. On NW, at least, they took the age limit seriously.