What about forgetting your 20s? I can barely remember what I used to eat before I learned how to cook. I assume that it was a lot of macaroni and cheese, pasta and mashed potatoes, but I can't really remember.
Any memories from highschool and such I know for a fact are second hand so I suspect maybe some people don't realise this - I'm more willing to believe in how easily we make false memories. I remember things as told to me by others even if they told me about it long ago. Riddle me that. I don't have amnesia, learning difficulties etc. Maybe I should keep a diary but it's a bit late for that.