I believe the reason is whenever you remember something specific, the neurological pathway to that piece of information gets stronger. For example I was taught

I believe the reason is whenever you remember something specific, the neurological pathway to that piece of information gets stronger. For example I was taught parts of the Quran between the ages of 8-14. I still know the verses because I use them daily (22-16 years later). The pathways are strong. It may also simply be that your brain learned to simply retain what seems important at one stage and disregard everything that looks alike. Example: in your eighties you can easily remember the time you had your first kiss if you only kissed 3 guys/gals in your lifetime, but if you kissed every last person at the party every Saturday night, then you probably will forget.