I also find less time to exercise as I grow older.
However, for the same level of exercise, rest, and study, I can't tell if there is a difference.
I also have more to forget, so I can barely remember entire arguments in philosophy that I had down cold 10 years ago, but even at age 20, you may not remember much from when you were 10.
Want to improve your memory and brain function at any age? Exercise. [0] Want to add new neurons (neurogenesis), exercise. [1] Want to improve your neuro-plasticity? [2] Learn new skills.
It's always bothered me nerds/tech-heads optimise their lifestyles for time, not realising fitness, novelty and rest can be used as a strategic weapon to getting better at what you do. I'm not being critical of the OP, I'm just observing what I see in my own tech circles.
Reference
[0] http://www.health.harvard.edu/blog/regular-exercise-changes-...
[1] https://www.dnalc.org/view/848-Exercise-induced-Neurogenesis...
[2] http://www.nature.com/nature/journal/v427/n6972/full/427311a...
In that sense, my memory is excellent, now I can remember a lot from 20 years ago.
Yep, definitely aren't as sharp as when I exercise more. And come to think of it, there was a good 13 year period where I was cycling a lot just to get around and now I drive to work every day.
I don't think having a smartphone in my pocket helps either, I tend to pull it out and read HN for 30 second periods during the day, then lose track, somewhat, of where-and-what I was doing.