I'm going to go further and say older people (particularly women) aren't cast as much because chances are they won't apply. I am over 40 and even though I love the show and think I probably could tough out the survival aspect, I don't see the point in going all that way just to be the first boot of the season because some douchebag alliance of 20somethings decide I'm a physical liability (which compared to them, I am).