There seems to be a growing acceptance that the brain must be kept fit just like our bodies. For me it comes down to a standard principle that I feel humans and other species have to adhere to. Use it or lose. Basically every thing that you can do is something you're body has to build and maintain during famine or feast. Your body is in a constant state of preparing for a famine whilst keeping you in the condition that you need.
Take this article I came across
My experience is that your body won't make a part of you strong unless you do something to push yourself close to or beyond your current limits. Take running for example. To be able to run further you need to challenge the parts of your body that limit how far you can run. Some people feel they just need to run further each time they run but that gets tough once you get over say 10 miles.
As I've blogged about earlier understanding how glut4 influences your ability to ran far helps you understand better how to train.
While we are young it doesn't seem to take much stimulus to get our bodies to improve or at least maintain themselves. A game of tennis every couple of weeks can be enough to keep some fitness and a few things that tax your mind, a quick presentation or applying for a loan might tax your brain enough. Even playing a video game may help if it gets the right parts of your brain working.
What I see in people more senior is that when you age it gets harder to trigger this response but it's still there. I've seen 80 year olds pop in to the gym for a quick work out in preparation for their ski-ing holiday who appeared and were as alert as many people 20-30 years young than themselves. I've seen the same in 60 year olds and what has always stood out is that they exercise both their mind and their bodies regularly. Don't get me wrong they know how to relax too. They aren't all work and no play. But they realise they need to keep their mind body and soul active now if they want to be able to stay active in the future.