Unless you're really having cognitive decline, you can probably pick it up again if you need it much more easily than the first time. We forget stuff we don't u

Unless you're really having cognitive decline, you can probably pick it up again if you need it much more easily than the first time. We forget stuff we don't use, so forgetting maths probably means you've been doing something else with your life. Maybe that something else was better than repeating the same maths over and over again? I have repeated some of the same maths for 10-20 years in my work and it's depressing in a different way - I'm not improving much. I'd be happy to forget the old and learn some new. Knowing about eigenvalues doesn't make me feel particularly fulfilled. Any 20-year old physics student can do that too.