The years are kinder to men, but not because of looks.
Physically, youth is attractive to both genders. Young girls are not going to be lusting over that "after" photo of Richard Gere. You can be a good looking 40 year old man in a dance club and still have a physical handicap compared to a good looking 25 year old.
The reason older men can theoretically have an edge over younger guys is because the traits associated with (successful) older men--maturity, confidence, good social skills, non-reactivity--are more valued to women than looks. Most guys in their 20's don't have these skills, and the girls their age are already a bit ahead of them socially, and know how to spot these desirable traits.
Now, if you're 40 years old and are still socially awkward, you're gonna be viewed as the male analog of that Kathleen Turner photo. Young guys can be AFCs and still get laid frequently, but as you age, you need to learn some game because the HBs will be less forgiving.
As for women--the years are bad to them in regards to being picked apart by the MM forum at Sosuave, but in real life, they're STILL going to have tons of options through their 30's and 40's. If a woman keeps herself in shape, she'll decimate the competition in her age bracket. The statistics that state that unmarried women over the age of 35 are unlikely to ever marry are skewed by the fact that many of them were against or unsuitable for marriage to begin with. In reality, a fit woman over the age of 35 will still have a ton of options.
Now, the caveat is that the "cougar" thing works out to be a lousy deal for women that are looking for long term commitment. No matter how they package it, there's still no ignoring the elephant in the room: many of the younger guys they date will want kids after her childbearing years are over. The biological logistics just don't allow for such a relationship to last unless both parties are 100% certain they don't want children.
Men, on the other hand, don't have to worry about this, and the only real drawback to dating younger is a lack of common experience.