I remember it being described a while a go as a triangle with three points - Time, Money and Enthusiasm. When you're young, you have lots of Time and Enthusiasm but no Money. When you get a bit older, you have the Money but not the Time. Eventually you have the Time and the Money but have lost the Enthusiasm!
Intersting statistical note is that, whilst GA activity is decreasing, helicoptering, microlighting and parachuting have all been growing.