The problem is with actresses that never learned how to act (as distinct from how to shake their booty)
There are a few who kept their careers past 40 (Katherine Hepburn being the biggest example), but for the most part their looks are all they have going for them
Consider that most of the time, the woman of the couple decides what movie to go to. Women really aren't going for aged actresses either
Actually, the really successful actors become movie producers (Arnold, Stallone, Mel) and star in the movies they produce.
A producer is the guy who does the business end of a movie... purchasing a story, securing financing and hiring a director.
Women actors, for the most part, think somebody else is supposed to handle all the business stuff and just give them plum parts.