It sounds as if the problem was that you were related to the supervisor (which is a very bad idea and which most companies do not allow, for this reason) and not because of your age. But when your supervisor, regardless of who it is, tells you to do something, it then becomes your job. Unless you have a legally binding and enforceable contract that specifically says that you cannot be asked to do anything other than x duties, your job is what your supervisor says it is.

Your problem is that you shot yourself in the foot. You *might* have been able to pull off an age discrimination complaint based on her "because you're older" statement IF you had not refused an assignment. By doing so, you handed her a legal reason to fire you on a silver platter, and now you'll have a much more difficult time proving that it happened because of your age. I'm not sure it can be done. It was less than a slam dunk to start with - you may have killed it outright.