You see it works like this:
Looking at a 15 year age difference,
If the female is 18 and the male 33, that seems to be robbing the cradle a bit.
If the female is 25 and the male is 40, then that seems more exceptable.
If the female is 30 and the male is 45, then that seems even more exceptable.
If the female is 40 and the male is 55, then that seems very exceptable.
If the female is 50 and the male 65, then that to is very exceptable.
So the only real issue here is the fact that the girl may be too young for her partner.