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.<p>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.
Some of the more creative people I've known seem to be the same ones that blur the lines between work and play. They'll have an interest in something and dive into it having a great time. They're constantly thinking, analyzing, enjoying what they're doing, looking at every angle of doing something. (sounds like Apple?) They're not afraid of much of anything either, including some failures (which they also learn from). They're not going through some memorized motions, they're heavily involved.<p>I guess that's where they play side of it ties in... heavy involvement, and the happy-chemicals that go with fun/play help to more easily form mental pathways.
Disliking what you're doing, being completely bored/mechanical or having excessive levels of anxiety tends to shut one down mentally.<p>Work hard and play hard