I believe the silly little rule for men is 1/2 your age + 7.
If you're 20, 20/2 = 10 + 7 = 17. Go find a 17 year old or older.
If you're a 24 year old female, in reverse it'd be 24 - 7 = 17 * 2 = 34 So don't go looking for guys older than 34
The rule is silly of course, but it makes a little bit of sense sometimes...