As captwannabe pointed out there is new legislation coming out to try and stop age discrimination. I don't think this will make any difference what so ever as it is so easy to discriminate. An airline are not going to turn round and say, sorry you didn't get the job cause you're too old. They'll simply say, there was someone better.
Being too young seems to have its dissadvantages too. I don't know if it is insurance problems, or the fact that airlines don't want some 18 year old walking out of the flightdeck, but getting employment with an airline under 21 seems difficult.