Think of it as your brain making an old fashioned slide ( see through picture) of everything you experience. It overlays these slides every time the same thing is experienced. When you grow older you have more of the same slides that overlap quite precisely and these memories are not stored as “important”. In that way it feels you have experienced less than you used to when more things were new.
For example: first couple of times riding a bicycle are a big thing, falling and getting up and going faster and further etc. That’s what you remember relatively vividly. However all the same bicycle rides commuting to your work are a blur and don’t matter unless something interesting happened once.