As much as it is every middle aged man's fantasy to tap a young, hot girl, with all clear conscious, I would have to turn her down. As a moral issue, I commited adultery once in my live and swore I would never do it again. Also, I'm the type of person that won't cheat on a GF, either.
AS for her age, I refuse to bed anyone that I'm old enough to have fathered. Anyone under 25 is off limits to me. I may flirt and joke with them, but that is as far as it goes. However, if this happened to me IRL, it would be one hell of an ego booster.