The best example I ever saw of someone realising that he was past his use-by date was in the UK.
I had not long had a DC3 TRE and was renewing an old guy's proficiency check. No simulator, of course. The weather was absolutely fogged-in everywhere, but off we went, because that's how it was done back then. Engine failures he nailed. Altitude and airspeed he nailed. But as soon as we got on to NDB work he became completely disorientated and eventually so lost I had to talk him back to the home airfield. Wherupon, becoming visual, he pulled off a perfect landing.
At the debrief, in deference to his undoubted experience and standing in the airline, I formally advised him that while I was not going to blot his records with a formal 'fail' notice, he was grounded until he did the check again. He did not even give me the chance to follow up with the offer of a different Check Captain; simply got up without a word, walked out, mounted his bicycle and rode off into the mist. Never to be seen again. Rather sad for someone to end a career that way, but he knew....
Now I am about the age he was at the time. I only hope that I can make the call before I get to the stage of self-doubt that he had reached. Scary thought.