Keep learning new skills, keep practicing and applying. Gardening is becoming much more important as retirement beckons. Dancing and lessons beckon when I have the time. Vary your cooking repertoire.
Reinforcement by practice of important knowledge and skills is useful. Forge new neuronal links, to replace those that fail or are pruned from lack of use.
Monitor what you struggle with, and practice if it's important.
Things fail, and everything degrades, but you can prolong your capabilities. The better you are, the more you practice, the longer you have.
Stupid example: to stop my back from playing up, I've started doing sit ups again. 200 yesterday, a fraction of what I was once capable of, but more than most people will ever manage.
The more you start with, the more you use it, the longer you maintain the functional level you need.
The uncharitable amongst you may consider thats why we see so many footballers with Alzheimers.That was an attempt at humour, but retiring at 35 and not needing to develop new skills may come with consequences.