It's the idea that women "lose" value with age, because they are primarily valued for their looks. Whereas men "gain" value with age, because of their accumulated wealth. Hollywood plays into this trope all the time - as noted by the fact that the leading men get older while their costars stay the same age...around 25.
While I'm sure there are loving couples where a 20 year age gap exists, it certainly isn't as common as Hollywood portrays it. Data shows that for most people, the average age gap between couples is + or - 5 years.
I think, and this is not an original observation here, that Hollywood shows an astonishing lack of imagination, and I suspect it's because it's run by the simplistic frat-boy mentality that never matured or evolved much past their 20's sense of entitlement and narrow mindedness. Tina Fey has commented on the misogyny by saying that Hollywood labels any woman as crazy if "she keeps talking when no one wants to f**k her anymore."
European films and media venerate women like Kristen Scott Thomas, Julliete Binoche, and Ines de la Fressange. Of course these women are more gorgeous than the average woman; that's part of their career. But they are also allowed to age and be seen as simply gorgeous...not "still" gorgeous.
It would help everyone to see women in the media who were shown as active, interesting people, worthy of respect, without it always boiling down to how they look and how offended the public seems to be when a woman shows up in the media and DARES to be anything but attractive.