Or at least some research suggests that: http://www.dailymail.co.uk/sciencetech/article-1162052/Old-age-begins-27--scientists-claim-new-research.html
To summarize, researchers suggest that mental ability peaks at 22 and starts to decline at 27. Presumably experience counts for a lot, and allows for improved performance for many decades afterwards, but the actual peak of our cognitive abilities comes much earlier.
I don't find this surprising, actually. Einstein made his biggest breakthroughs while still in his 20s and chess masters arrive at their exalted status as teens or young adults, not as people nearing retirement.
I've noticed that the intricate, detailed wargames that fascinated me as a young man no longer hold as much attraction now that I'm much older. I thought that I'd simply lost patience with them, or "matured" in my tastes or something like that. But it may be something as simple as the fact that I'm not quite as sharp as I was back then for that sort of thing. It's too much work now. Back then it didn't seem like work.