I think that this is a better and more constructive article than the various other age-related articles drifting around. It may very well be that the level of fluid intelligence is the real issue, and that this level is partially inversely correlated with age because, as time goes on, more and more people stop playing, and few ever start playing again once they've stopped.
Many people probably stop play-like activities because they consider it to be wasted time, much like sleep, and so it gets the axe when time is scarce. Perhaps if they realized its importance, they'd make time for it.
I think a culprit in age-related decline of fluid intelligence is Careerism.
I don't mean the idea that you should take your career seriously. That would be lower-case-C careerism. What I mean is the idea that you should evaluate every aspect of your life from a point of view of "will this get me a promotion" or "will this get me a better job?"
That not only causes you to stop playing, but in my experience it kills all creativity dead.
Many people probably stop play-like activities because they consider it to be wasted time, much like sleep, and so it gets the axe when time is scarce. Perhaps if they realized its importance, they'd make time for it.