The rule to define the youngest that a romantic interest can be before the relationship is indecent.

This is a very old rule of thumb: it is the calculation applied to a man's age that provides the age in a woman that he is most naturally attracted to physically. (Things change of course once you begin to actually TALK! :-o )

In recent years it has been misapplied to mean the lowest age that would be considered "decent" for him to date.

This new interpretation of the rule is actually quite bizarre when you consider the remarkable range of maturity among adults of both sexes at all ages.

Bullshit formula that middle-aged me use to justify porking coeds. If they were consistent about it, they would also advocate being with women using the inverse of that formula: Twice-your-age-minus-seven.