If you’re interested in maintaining your brain, find people with whom you can exchange ideas. Not everyone has the same interests, and that’s okay, but find those people with whom you can really dive into the weeds and discuss the details of a subject.
It’s like playing ping-pong with concepts. Someone lobs a thought onto the table, your brain responds with a related idea, and the conversation is built on following the chain of concepts that emerges. Fun stuff!