For the airline industry the retirment age is built into the retirement system so we are loosing experience. The pilots just go on to a second job or enjoy golf a lot. Kind of like firefighters or cops.
The ADA does have very job related clauses. Physical requirements need to have a reason. At 6'4" I was too tall to be an Air Force pilot. The planes were built for shorter people.
For bus and train operators the requirements need to be based on work skills. Lifting, bending, crawling under to reset emerg. stops how many LBS that need to be lifted etc.
I worked with a guy once where the company moved the bus seat back for him so his belly would clear the streeting wheeel. Next they had to put blocks on the pedals so he could reach them. So much for reasonable accomodation.