how appropriate for this thread - in my mail the other day was the Officials Quarterly by NFHS - an article titled "why don't they retire - by longtime football official Jerry Sulecki. Although he doesn't give a clear decision on how to answer this - I agree with a quote "Officiating is not something I do - it's what I am. Officiating defines and dominates my life". Maybe thats why its tough to give it up. It doesn't seem like anyone side of the equation; assignor, association or official can come up with an easy answer to determine when it is time to retire.
There are a couple of older (70+) officials in my association and they still TRY to officiate. I am on the younger side of my group and have been at this for 13 years. I am poised and ready to take on more challenges IF these officials would/could realize that their time has passed. I worked with one last year and he told me "that he had cataract surgery, a bad hip and would not be switching on any fouls" - that may work with him with the coaches but it certainly would not work for me as I am still earning coaches respect.
On the flip side
I worked with an official last year who let it be known that it would be his "farewell tour" as he felt that it was time and wanted to go out on his terms. I left that game with the utmost respect and admiration for that official AND thinking "thats the way I want to leave the game".