I think you're misconstruing what happens. When a woman gets older or has children, very often her hair thins dramatically. A lot of women lose handfuls of hair during pregnancy. Long, thin, broken, straggling hair is not attractive. So women in their late thirties and forties often have to start cutting the thin hair. A woman like this one who still has luxuriant locks in her forties is rare.
As for me, In my middle age I'm fighting it out to the bitter end because Mr. Fairview would kill himself if I cut my hair. It's up to the shoulders now, though.
There is an old adage that no one over the age of 18 should have hair longer than shoulder length and that hair should be cut progressively shorter as a woman ages. The reason is that long hair tends to “pull down” the face, making a woman look older. Also, long hair takes a lot of work, and when a woman acquires a husband and children, something has to go. Sometimes it’s long hair. That said, methinks I hear a major tone of bitterness in your statement. What short-haired woman hurt you so badly, that you have to apply such a sinister motive to all women?