Half your age plus 5 is the mathematical equation that NFL QB's use to determine the creepiness of a dating relationship.
Take this example for the Sanchez Rule:
Person A: Is 24 years old.
Person B: is 17 years old.
24 / 2 = 12
12 + 5 = 17
This relationship is not creepy because person B's age is equal to 17.
Therefore date someone who's age is at least half your age plus 5 or you're creepy.