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.
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.
Kate is 20 and dating a 26 year old named John. 2(20-7)=26. This relationships satisfies the inverse of the half your age plus seven rule.