I know the feeling
That moment happened to me a few years ago, when, as a consultant, I responded to a call from a university.
As I reached the double glass door entrance, a student on the other side noticed me coming and casually reached for the door, holding it open for me. What floored me was just how automatic and casual the gesture was, as if it was a given that that was what he had to do. Some people would call it education.
I call it my "now you're old" moment.