From New York Times:
Research shows that longheld views have been prove wrong. For example it is not correct that 40 percent of brain cells are lost when the brain ages.
Adults retain the ability to learn. The brain, as it traverses middle age, gets better at recognizing the central idea, the big picture. If kept in good shape, the brain can continue to build pathways that help its owner recognize patterns and, as a consequence, see significance and even solutions much faster than a young person can.
My personal note:
I have collected in my blog some evidence and scientific papers proving that our brain retains the ability to learn and adapt. I believe that if we imagine our brain as a muscle we can understand the problems arising. If you do not exercise then you ask for trouble both physical and mental.