People with age amnesia forget that they've have ever been kids, teenagers, or 20somethings and start to criticize the current young population as if they themselves were never a child, teen, or 20something.
People suffering from age amnesia suffer from forgetting they themselves were once teens with raging hormones or noisy little bastards. This comes with a steep misunderstanding of the young, as sufferers of age amnesia have never been a child.
20something - "You were a hippie back then, didn't you do the same?"
Middle Age Man - "IT WAS DIFFERENT BACK THEN!"
20something - "Whatever man, you got that Age Amnesia"