1) Long hair highlights the face - bringing it added attention.
2) 99.99% of women past 40 do not have particularly attractive faces (nor particularly attractive figures).
3) Thus, it makes sense to cease trying to bring attention to their faces.
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The same basic logic is why you don't see a lot of women in their 40's and up wearing mini skirts, bikinis, . . .
When you think about it for a second, it really is quite a sensible adaptation to the changing reality of their faces/bodies.