Well the way memory works is that it allocates certain clusters of neurons to storing information. When you're young there's a lot of blank space that you can store stuff in but as you get older you start having to pick and choose as more and more brain space gets taken up.

Here's a cool video on the subject: https://www.youtube.com/watch?v=X5trRLX7PQY Fun fact: because of how memories are formed in chains you can tell if you're on the precipice of forgetting something if you try to recall it and you start trailing into another memory. You can experience this for yourself by trying to recall the beat of an old song and note when it starts morphing into the beat of a newer song. It's also worth noting that every time you recall a memory you destroy the original and rewrite it, bringing it back to the top. That little asshole is like 90% of the reason why our memories suck so much shit and are so prone to outside manipulation.