You have an excellent point! I think alot of people see men as getting more sophisticated/sexy with age, whereas older women are seen as "grandmas" (which is not sexy).
Actually, in alot of "older man" cases, the woman is after the $ too, so it's not just young men that are viewed that way.
The grandma thing is about all I can think of, or maybe men just age better and that's why they are more accepted? I often wonder about these things myself.
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Muthu