Legally binding agreements are one thing but if the law of the land changes then they are nullified. Taking Shytorques case if there is a union agreement about permitting a place to smoke within a smoke free company legislation that bans smoking in a public area nulllifies that agreement.
However this thread is about legistrated enforced retirement from a profession, not from a job.
The argument is that any pilot who desires to follow his profession, if he or she is proven to be fit enough and skilled enough, shoud be allowed to do so as long as he or she likes.