Years ago, I read about a theory that stated the 'mind' only has a finite memory capacity. So at aged 10, e.g., it looks like this
|_|_|_|_|_|_|_|_|_|_|
Each year gets condensed into an even smaller space as you get older - the longer you live, the more the brain condenses the information - and it is not stupid, it only saves the 'memorable' stuff such as the first time you had sex, got drunk, broke a leg etc.
So by the time you are fifty, you have 50 years crammed in that small space - and a lot of info is dropped.