A longing for the good old days, whether or not they were very good. Or:
A tendency to think that if you were twenty+ years younger, you would be happy -- as if you actually had been happy when you really were twenty+ years younger.
Teacher: "Back in the 80s, I ruined a bunch of blazers by rolling up the sleeves like Don....."
Student: "Get your pillow, Mr Nostalgia rides again."