I think perhaps the best way to answer how to stay mentally active with the question, posed to me, that got it through my own head.
Start by asking yourself this question:
Why do you think that time seems to speed up as we get older?
Now stop, and really think about that question. Don't skip ahead. Think of some possible reasons. It's a phenomenon that we all experience. What's the cause?
When we are preschool age, time seems to last forever. As we get older, the years start rolling by more quickly and I've been told by more than one person in their 70's that it never slows down, it only speeds up.
Why?
The answer is actually that we've 'been there, done that'.
When we are young, EVERYTHING is new. When we are experiencing something new, time seems to slow down, especially because we are enjoying it and taking in so much.
As we get older, our brains start to filter out what we have seen before. You know that feeling when you drive home and don't remember driving home or anything along the way? Boom. There you go. Your brain just ignored the trip.
Have you ever gone on a three day trip to some new place and felt like your were gone for two months? It's only three days, right?
Well, what you effectively do when you experience new things, you slow things down again. Your mind stays active, and here's the kicker...your brain actually figures out better, more efficient ways to move information around the synapses in your grey matter. No, that's not 'mumbo jumbo'. The reverse is also true, which is why once someone experiences depression, they are always susceptible to it. The brain learns the path to certain behavior.
This is why confidence mantras work. Want to make your mind more powerful every year from here on?
Try new things.