Why Is the London Underground so Full of Young People?

I like subways in general and I love the London Underground but I must say that compared to New York, the London crowd seems generally younger, more fast moving, more likely to run down geezers like me.  This is fine, it keeps me hopping, but I'm wondering why?  My first guess:  London Underground travel in general is probably cheaper than New York, adjusted for exchange rates, earnings, whatever. Since when I say "younger" I probably mean "richer," it may be that the hunble folk here just can't afford the tube.  And second--I like this one better--New York still has flat fees, so humble folk who live all the way to the end of the line can still afford to travel by subway,  Any such person who lives beyond, say, Zone 1 in London, is going to find another way to travel.

But wait, you'd think this would sort itself in the price of housing--flat NYC fares should raise the prices of real estate in the outer suburbs.  And I really haven't thought through the opportunity cost of taking the bus in either place.  So I guess I am, you know, like just as ignorant as when I started.  Are there any other good theories?