This question keeps popping up. I'm not a geezer and maybe I'll feel differently about this when I am, but I would ask this: Why would you ever have to stop? If you can squat 315 in your 60s, why not go for 320? Then 325? So on, so forth. Of course we'll all reach our limit someday, but why put an arbitrary limit on ourselves?
Part of strength training is the quest for improvement, I think. That has nothing to do with how long you've walked the planet.