I for one could never have sex with a woman younger than my daughter.
Of course shes three.......
But seriously when I was younger, I was completely turned off by older woman. Whats interesting, is that while I've gotten older my 'upper' strike zone has stayed the same years wise as it was. It just goes up 1 or so every year. Obviously its not a hard fast rule though.
On the flip side, unless the young woman was of greater than average maturity and intelligence I'd be reluctant to swing with a woman half my age.