AFAIK nobody has a clear idea of which cognitive abilities degenerate due to normal ageing AND respond to exercise (other than N-Back helping short term memory, perhaps).
I'd start by looking at the standard components of cognition. Start with the sub-metrics in the Wechsler IQ test. See what specific mental skills you might want to maintain or improve.
Then design your own tests to assess each skill and your own mental exercises to see if you can strengthen your mental muscles. Positive feedback is your best guide, IMHO.
If you are interested, google "gwern dual n-back", there is a lot of info there. In the end, it seems a lot of people disagree with Jaeggi that n-back training carries over to iq, which is unfortunate. Still, cool post.
I'd start by looking at the standard components of cognition. Start with the sub-metrics in the Wechsler IQ test. See what specific mental skills you might want to maintain or improve.
https://en.wikipedia.org/wiki/Wechsler_Adult_Intelligence_Sc...
Then design your own tests to assess each skill and your own mental exercises to see if you can strengthen your mental muscles. Positive feedback is your best guide, IMHO.