A convenient rule to define the youngest that a romantic interest can be before the relationship is indecent. Widely referred to and used, the rule has merit for usefulness. The full version states: "if you halve your years then add seven, you'll have the youngest decent age for a partner; if you double your years then subtract seven, you'll have the oldest decent age for a partner".
However it's somewhat flawed thanks to the poor mathematical ability of its authors -- it gives a different result depending on whether it is worked out for a younger or older partner.

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A mathematical equation used to find the youngest age a person can date without being considered a disgusting, cradle robbing freak. (A/2)-7=Y, where A is your age and Y is the youngest you should date.

Conversely the rule can be inverted to find the oldest age a person should date. 2(A-7)=O, where A is your age and O is the oldest you should date.