Yes, we all understand that people stereotype. I think you're getting bogged down in the details and missing the overall point, which is far more generalized. The above story is meant to be short and clever, and to do so it glosses over a few things that you're meant to pick up contextually. The point it's trying to make is this:
20 year olds tend to think people are watching, judging, and remembering their every move and are concerned about it
40 year olds still tend to think they're being watched and judged, but don't care
60 year olds realize that everyone is too busy thinking about their own situation to really pay much attention to others beyond superficial stereotyping, and that unless you affect someone deeply with your presentation, they won't likely cling to previous judgements of you.