Battlefield 4. Laugh if you will, but playing this game (or games like it) against other humans is an incredible mental exercise. There is little downtime and twitch does win the day. Interestingly, I usually only play when I'm too tired to code productively anymore - and playing brings me wide awake.

It's amazing to me when watching others play how they miss cues I can now see. Or how my mind has subconsciously optimized where to aim in a heated firefight depending on the gun I'm using (sometimes sternum, sometimes head - I more notice I'm doing it as opposed to actually doing it).

Plenty of good advice here, but high-speed mind activities should be included in any advice of "staying sharp" (i.e. Battlefield, RTS computer games, iphone brain games I suppose)

(relevant: Starcraft performance factored by age: http://journals.plos.org/plosone/article?id=10.1371/journal....)