FuturePundit has an interesting entry on working memory. It seems possible that, in addition to raw intelligence, or processing power, various other factors go into your mental performance, including working memory and long-term memory. To have complex thoughts you need to hold the various parts of the thought in your memory while having it. And to develop sophisticated skills you need to remember them for an extended period of time.
For anyone, no matter how smart or successful, one or another of these is probably the limiting factor in mental performance. If you knew what your own limiting factor was, you could focus your attention on it. This would be the most effective way to get better performance out of your brain. And, even if there are hard limits to improvement, you could make decisions about the types of tasks you would be best at and try to get work that lets you do those tasks most.
And, as time goes by and you get older, it would be useful to know where you are declining most. You probably worked out ways of doing things when you were younger that suited the mental skills you had then. You are not likely to let these habits go, even though now, with a different set of inherent abilities, they are suboptimal.
Note, I say "useful" to know. Not cheering or delightful. Cognitive decline seems inevitable. But you can probably work out ways that that decline hits actual performance as little as possible. As in war, victories are nice, but successful fighting retreats are what take the real skill. As much as possible, our goal is to live to fight another day.