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.
26-year-old Barbara waited patiently until Jack turned 20, fulfilling the half-your-age-plus-seven rule, before pursuing him romantically.