"Of course, that expected decline may not be very reassuring to those who dread the idea of getting older. But you can do things to improve your chances of aging gracefully."

One thing might be to avoid early retirement.  My wife has noticed that she's "just not as sharp" as she was when she was juggling work, house, family, etc. on an hourly basis. It's not severe, but it's noticeable. Use it or lose it.