Few minds wear out; more rust out.

Disuse does more damage than hard use. A mind kept at difficult things stays capable far longer than one protected from effort, and decline from idleness is quiet enough to pass for ageing. Nobody notices the day they stopped learning anything new. They simply find, years later, that new things have become harder.

Take on one thing this month that you are currently bad at and would have to think about. It does not need to be useful, only genuinely unfamiliar. Difficulty you choose is cheaper than the stiffness that arrives when you never choose any.