This is where the term got attention!
Demi and Ashton were pretty much the first couple to bring the idea of "cougars" (women dating noticeably younger men) to the public eye. Although it has been happening for some time now, they were the first celebrities to do it and therefore make it widely recognized. The term may even have been around before their coupling but it was not a household world as it it getting to be today.
There was such a hubbub over this particular relationship, Courteney Cox has created a tv show based on this idea. However, hers is less about specifically dating younger men, and more about an older woman being single. Most women of her age (and level of attractiveness!) are taken. Jules, the main character in the show is attractive, outgoing, and can pretty much date whoever she wants.
When people talk about a "cougar", it is less that they are talking specifically about a relationship in which the woman is older and more about the fact that an older woman can date whoever they want. If a woman is dating a man who is noticeably younger than she is, she has got to be secure in who she is, confident, and attractive. The man that she is dating must obviously be mature and settled.
In today's modern world, getting married and starting a family in your early 20's is becoming less of a "social norm." These women are wanting to have careers and to have professional accomplishment as well as fun with their friend before they settle down and begin their traditional role. With modern medicine and technological advancements, there is no reason that women cannot have children later in life, so the time threshold is widening, allowing women time to do things that they desire before beginning their family.
A "cougar" can often be a case of being in the right place at the right time. You can't always choose who you fall in love with. An accomplished woman can decide that she is ready to settle down and then happen to find a younger man who wants to settle down and start a family earlier than most. There's nothing wrong with this-it's important to be happy with your mate for your own reasons. Date who you want, discriminating or allowing other people to judge based on age is the same as deciding not to date a great person because of race, socioeconomic background, or monetary issues. Aren't these all isues that we're trying our hardest to dissipate? Think about that.