How Old

[oddball]

I suspect the real litmus test for this is do you trigger an "ick" response, and that is more likely out of bad hygiene than age.
As you become less limber you may need to let them do more of the work but as long as you're otherwise a good customer: respectful, don't try haggle, etc... they'll probably rather that than some young "stud" who at best annoys them, at worst thinks they should be thrilled to be in his company.
Besides, they're probably a bit relieved if you can only manage a single "main event" and then fall asleep/just want to talk and cuddle, just don't live in denial that you can go another round and it's somehow up to them to make it happen.