Like others already mentioned: hair loss, high school girls, not wanting candy anymore.
Not knowing who a lot of famous musicians and actors/actresses are.
Starting to also want to avoid getting drunk. None of my friends seem to want to stop drinking yet, so I might be alone on that one at 28 years old. The hangovers are not worth it because the entire next day becomes a waste while the effects are slept off, headache nursed, and body rehydrated. When I start getting ready for work on Sunday night I feel cheated out of half my weekend. Fucks with your sleep schedule, and motivation to do chores and exercise. Basically throws your life out of whack for at least 2 days and not getting certain things done can have ripple effects that last throughout the week, like being annoyed in the morning that there is no clean laundry to wear because I didn't do it over the weekend and it's a pain in the ass to have to go to the laundro mat and drop off & pick laundry up in the morning before work.
When they start making remakes of movies that you liked as a kid, and when you date a younger girl (I was 27, she was 22 in this case) and you turn to a movie you liked in the past on TV and try to watch that and she goes "coome ooooooonnnn, I don't want to watch OLD MOOVIIEEEESS!".