Do (most)women just switch because they crave a challenge?

I was thinking, since a lot of women just want what's more difficult to get/obtain. Does that mean they like bad boy's in their younger/prime years, because they are more difficult to get. And in their later/decline years they want a good guy, because they are more difficult to find in their older years?

Since the good guy's that were around in their younger years, will also change and be more jaded due to negative life experiences. So there will be fewer of them by the time she is in her 30's.