This concept goes both ways. Here is a simple rule that my cousin, Mary Therese, taught me about dating and the social acceptability of age ranges.
GIRLS: (Your age - 7) x 2 = The maximum age for a guy you would date.
GUYS: (Your age divided by 2) + 7 = The minimum age for a girl you would date.
Agree? Disagree? I think it's a pretty good rule of thumb if you are between the age of 15-30.