I think two aspects are at play here.
1) Men remain 'sexy' longer than women. Not entirely sure why. Suspect it has to do with how power relates to sexiness. Most 18 year old girls don't find 50 year old men sexy - but power is an aspect of a man's sexual attraction. If a man stays in good physical shape, is successful and (important!) seems capable of talking intelligently, he's still able to play a romantic lead. Sean Connery (sp?), Harrison Ford and Cary Grant all come to mind. Young women - say, from 18-35, could imagine falling in love with such a man.
Young men, from 18-35, can project themselves as someday turning into a Cary Grant - tho at 45, I've got to admit my odds seem poor ;>) - but men aren't as interested in romance as they are in sex.
That doesn't mean a woman becomes undesirable at 40 - either Hepburn, Sally Field or Michelle Phieffer (sp - sorry) are examples of beautiful women who remained beautiful past 40 - to men who are past 40. But the normal target audience is the 18-30 male: guys who define good beer as 'free' and good wine as one with a convenient screw top.
When I was in high school, a very pretty but very quiet girl shocked a group discussion on sex by commenting simply, "All guys want is to get in your panties!" Since all of us guys sitting there wanted to get into hers, it was kind of hard to deny. That's the target audience - the ones who buy tickets - so Hollywood responds with young, artificially enhanced females. Very few men mature enough to appreciate a grown woman buy movie tickets - don't know if that is the chicken or the egg.
Bottom line to point 1: Girls want romance, and can imagine falling in love with Harrison Ford. Boys want sex, and cannot imagine getting it from Hepburn in her 50s. So men remain 'sellable' longer.
2) Many older roles require someone who has had success in some field for a long time in order to be believeable. For example - you need to send a highly experienced astronaut into space to save the world. How many woman astronauts were there in the 60s? What is the percentage of female fighter pilots? Army Rangers? You need a successful (and therefor evil) CEO - do you pick a woman or a man? One in a great while, a role comes along requiring a successful mother - the wife in National Velvet comes to mind, only 50 years ago - but they are rare. Many roles require a man in his 50s, even if there is no romance/sex involved.
Heh heh heh.... yeah.... that's a pretty succinct description.
It's simple. Men remain fertile longer than women.