The rule (invented by wise men but which should also be adopted by wise women) which states that half your age plus seven is the youngest acceptable age of a potential romantic partner. (Does not work inverted.)
Example: "C'mon, man, she's legal!"
"She's 19; you're 26. Just because she's not jailbait anymore doesn't mean you should date the girl. Don't you know the Age Rule?"
Number Example: 26/2=13. 13+7=20.