Why does Hollywood have an aversion to "older" actresses?

Why did Ann Sheridan’s film career peter out in the 1950s?

Because after Warners, when she was freelancing, she committed the one sin Hollywood could never forgive actresses for — she turned 40. And there was an entire generation of younger stars by then. All the stars who began in the 1930s had a rough time in the 50s. It’s still true today.

It is certainly no secret that quality roles for actresses over 40 are severely lacking beyond the typecasting as grandmother or disabled wife. I've posted my complaints about Hollywood on some other matters like making claims about top grossing movies and the overall lack of interesting movies. It makes me wish I was born about 50 or 60 years earlier, so I could have had the chance to work with Alfred Hitchcock, Howard Hawks or some others.

This is no secret, but my question is why the aversion? Surely it must be something beyond just the prima facie of men running things. I can't imagine if indeed it was just because men were running things, that they can't find "women over 40" to be more interesting. I mean not "all" men go chasing after the next pretty young thing that comes along after their own wives reach 40 do they? Personally, I find "older" women (I'm 32) to be interesting. One of my favorite actresses is Jane Seymour. I never did like the fact that CBS essentially threw Jane out like old dishwater ten years ago.

I don't know. Maybe I'm a modern man, yet most of the movies I watch were made before I was born. I am more likely to be watching Turner Classic Movies or renting a DVD than going to the theater these days. I find Joan Fontaine, Kim Novak and Grace Kelly to be fascinating. I find Britney Spears, Paris Hilton, Angelina Jolie and that boy toy of his to be NOT so interesting at all. Talk about a study in contrasts. But what do I know?

Perhaps I will follow through with my dreams of putting a movie on the big screen someday. Without revealing too much detail, it is basically an intergenerational story as told from a grandmother to her granddaughter (in her own house, not a nursing home) about life as a young woman. Now I need to write that script. Hopefully, it will come to fruition.