My old English teacher used to say that "you know what you like". OK, he was talking about poetry but I have found it relatively easy to remember the things I need to know about the activity I love. Even after a break of 23 years, when I had to do all of the exams again, I could remember much of air law and enjoyed re learning the things I had forgotten. Partly this was because I kept in touch by reading any aviation matter I could get hold of.
I still enjoy acquiring knowledge about all aspects of aviation which is one of the reasons I follow Pprune and reading about the experiences of others.