Now pushing 70 and retired for 4+ years, I don't seem to have any difficulties learning new or difficult things. There's some irritation at not remembering some of the prerequisites to certain topics that you once knew well, e.g., all the old trig identities. But after 50 or so years, that those memories are somewhat degraded is hardly a surprise. As others have noted, they're still in there, you just have to coach them out and polish the pathways. I seem to have more pursuits now than I actually have time for. Right now, I'm working through a calculus/diffeq refresher to prepare to go down the physics/electronics rabbit hole. I've spent some time playing around with Arduino and while it's fun and you can build a lot of things, it's mostly about what. And what and how were the dominate memes from my working life. Now I want to know the why behind the how on many subjects. And the math that has decayed over the ensuing decades is thus essential. For me, retirement has been like a new beginning. Live like you'll live forever. You won't, of course, but it sure as hell beats the "Waiting around to die" mindset.
