My theory has always been that women in film and television- it doesn't seem to be as much of an issue for stage acting- are trying to conform to the demands and biases of male television and studio executives, most of whom seem to find it intolerable to see women onscreen or on the small screen who are the same age and have the same level of awareness and life experience as the first wives they ditched- sometimes at great monthly personal expense- for the "younger talent" they felt entitled to trade up to, after they got wealthy and powerful in the industry. These men can't tolerate the idea of major roles going to women who are old enough, wise enough, to be on to them and their games. They can handle casting the young and pretty, or the too-old-to-be-threatening to their egos and delusion, but not the women who've known them the whole time.