Here's an excerpt from John Medina's excellent book Brain Rules.
When you learn something, the wiring in your brain changes … The brain is constantly learning things, so the brain is constantly rewiring itself.
My mom often says it's difficult for her to learn anything because at her age her brain can't produce new cells. We now know that to be false; thanks to Neurogenesis and Neuroplasticity, we can and do continue to learn for the rest of our lives.
Nowhere is this more obvious than learning a new skill — a musical instrument, a new sport or in my case, Cardistry. When I first started trying out the thumb cut I kept dropping the cards. Eventually my brain rewired itself to the point where I could do it slowly with half a deck. Now I can do it with a full deck (with either hand!) though there is still room for improvement.
If you really believe that you can't learn anything (because you're too old or not as smart as everyone else) then pick up a hobby that requires you to learn something new. And it doesn't have to be complex or require any special equipment. If Cardistry isn't your thing how about pen spinning or rolling a coin across your knuckles, two skills you can learn without getting up from your table? Doing so not only silences that part of your brain that says "I can't learn because INSERT-REASON-HERE" but replaces it with a more supportive statement backed by science: you can learn because that is what your brain was created to do.
Image of man holding cards by Elena Gattl. Licensed under Creative Commons Attribution 2.0 Generic.