+0
  1. Maybe it's because I'm getting older, but If I memorize something I forget something else. That's why people say you have to study when you're younger.
  2. Maybe It's because I'm getting older, but every time I learn something new, I forget something else. That's why they say learning is for the young.

I can't see why the writer uses but not so. Does "but" mean "so" in the context? It seems to be clear to me that A (I'm getting older) is the cause of B (every time I learn something new, I forget something else).

+1

"but" is there because of the wording "maybe it's because".

"Maybe/perhaps/possibly/etc. it's because/owing to/etc. X, but Y" is a set pattern of words that we use when Y could be a reason for X.