Interesting.
Simple answer: Physical decline of the brain starts at around 25. But with practice and experience players (e.g. in chess) can still get better despite that. It is obviously not a sharp decline until you reach a very high age.
see e.g. this research
http://news.bbc.co.uk/2/hi/health/7945569.stm
Side note on old poker guys:
At my local $100 Tourney there is a
86 yo guy who just crushes, because he plays a strat exactly inverse to what most of you are thinking:
He slowplays all his monsters. He fastplays all his marginal hands (don't know about correct semi-bluffs, though) and even uses second pair as kind of a semi-bluff. I don't think he is any good, but he somehow has figured out that most people fold to his aggression (and they still keep doing it)
And he figured out that slowplaying is good vs most opponents.
He limped Q8o in UTG once and barrelled twice with a gutshot + overcard, I called twice with top pair. On the river he was close to 3rd barrel, to which I would have folded, but luckily he gave up. I have been overfolding to him all the time before that hand!!!
On 3 other occasions he limped AA and AK at 10-15bb and took me out because I jammed (correctly) with a hand that turned out to be brutally dominated.