I have observed that as well. Some of it has to do with the simple biological reality that while older men are a "hot commodity" throughout their whole lives and still manage to date women 10-20 years younger than themselves, that option generally isn't open to older women. God is forgiving; nature isn't. From some older women couples I've known, I'd say that it's not even primarily about sex; it's more about companionship & avoiding loneliness.
Well put.