The interesting thing is how subtly middle-age and old-age catches up on you. I remember watching some kid ask a 90-year-old what it felt like to be old. He said, "It feels just like it damned always has. I'm only old on the outside"
I didn't understand that at the time.
I used to joke about TV shows as part of helping people out with learning programming and technology management. One day I'm in a room teaching something and I say "That's about as likely as Gilligan and the rest of them actually making it off that island"
Total silence. Nobody in the room knew what "Gilligan's Island" was.
That happening more and more over a few years' time, and finally I decided that I don't tell jokes based on TV shows anymore. For one, most of my frame of reference is in the 80s. For another, I don't waste my time with that much network TV. I don't think it has the cultural significance it used to have.
But there was never a bright line when it happened. I don't wake up one day and somehow realized I'm a middle-aged person. Life feels just like it damned always has.