I will agree it depends how young she is, 18yr olds look like little girls to me these days, but I have heard the rule for a man is half his age plus 7 years, which would put my age limit at 35, my GF is 37 so I'm not breaking that rule.
I just dont like double standards, Cher has a boy toy almost 50yrs younger, Richard Hatch has a boy toy 30 yrs younger, but it is "Tsk Tsk" for a straight man to do the exact same thing for some hypocritical reason.