Many people believe mental function decreases with age. Although there is good evidence we loose brain cells over time, my research indicates this can be minimized and the cells we have can be made more effective.
As I turned 40 I read a book about memory as we age. It turned out the reason most people feel they can no longer learn things is they no longer experience what is called "original awareness." When we are very young everything we experience through our senses is new--an original awareness. As we age we learn to not take in details to minimize our mental load. It turns out this can be reversed. We can selectively remember anything we want to remember--very cool!!
A few books you might like to read are:
The Memory Book by Harry Lorayne
Memory Fitness over 40 by Robin West
These teach fundamentals of memory and how we change with age.
To maintain as many brain cells as possible requires a good blood flow to the brain. Again this gets back to good nutrition.