Mianii of the Ursin realm is collecting data in a forum thread on the average ages of players. She has organized the information by class, and what she has found so far is pretty interesting. Each person that posts in the thread includes their class and age and she adds it into her calculations (eww math). Currently the average age of paladins polled is 22.88, while the average age of rogues polled is 16.24. Weirdly enough, mages average at 34.74, so yay for me being ahead of the curve for once.
Now before we start making conclusions based on these numbers, we have to realize she is polling forum posters, and that is a very small segment of the player population. I would love to be able to get access to this data from Blizzard, since I am sure they are storing demographics somewhere. If we had an accurate idea of the ages of the players then we could start forming generalizations about age, attitude, and class mechanics and behavior.