Quite a tricky problem too for those running clubs. Obviously venues that employ well known singers will use commercial reasoning. But for clubs where no money is involved but we become a group of friends, it is hard for the person who organises the callers rota (say)to drop someone who is becoming slightly forgetful. They probably have given a lot to the club in the past and need the club to support them now. But for the good of the club and its present members they need to stop.
A friend recently stopped singing with a BBC choir - they re-audition every year and he knew his voice wasn't as strong as it had been so he dropped out.