I suspect that statistical analysis comes into play in determining the age cutoff. While I know 80-year-olds who are completely ambulatory and 50-year-olds who need canes to get around, those are the exceptions to the rule. My guess is the client has looked at the work involved and, based on [most likely] insurance company statistics, has determined the maximum age that they want people doing this job.
You can always ask the MSC about an exemption.
Slightly off-topic but germane to the discussion of exemptions...a few years ago I was asked to shop a particular dealer of a German high performance sports car. I reminded the scheduler that I didn't have a car that qualified. I was asked if I could come up with a convincing "backstory" to explain my vehicle. I did, and successfully completed the video shop. The client was so impressed with the ease of my telling the backstory, that I was able to do more shops for them for the rest of the year. You lose nothing by asking.
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Have PV-500 & willing to travel.
"Answers are easy. It's asking the right questions which is hard." (The Fourth Doctor, The Face of Evil, 1977)
"Somedays you're the pigeon, somedays you're the statue.” J. Andrew Taylor
"I have never met a man so ignorant that I couldn't learn something from him." Galileo Galilei