As you age you lose some mental quickness, sheer speed. You can also become less open to new possibilities, more set in your intellectual habits, and less quick at learning new material. It is quite possible that I was more brilliant twenty years ago. I look back at my first book and wonder how I could have been as smart as the person who wrote that.

On the other hand, you can gain erudition and experience, become smarter in other ways. In fact, if you aren't doing this then you will have only the negative effects of the slowing down process without the added benefits. Imagine if you could play competitive tennis at a high level up to the age of 60. You would have an edge in experience and knowledge. If your physical decay was only slight, then you would beat everyone else. In scholarship, the equivalent of 25 in tennis is probably about 45: that's the age when your mental faculties haven't atrophied yet, but you've gained a lot of experience / knowledge. The good thing is that you can actually keep some of that mental flexibility and acuity and even increase some capacities that don't depend wholly on sheer speed and memory.