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.