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

The social rule defining the youngest age one may date without it being socially unacceptable (or illegal). To compute the oldest one could date without it being socially awkward, simply subtract seven from one's own age, and multiply by two.

To compute the acceptable age an otherwise inappropriately matched couple would have to be before they could date, find the difference in their age, multiply by two, and add seven to get the age the older partner would have to be.

And remember, always round up when computing the youngest age you can date, and round down for the oldest. You know, just to be on the safe side.

The lower limit on the age of a potential partner for a person over 14, by socially acceptable standards. Used as a rule of thumb.

The converse is twice-(your-age-minus-seven).

The following online calculator makes the math easy and also calculates how long until an UNacceptable relationship becomes acceptable:
tinyurl(dot)com/halfpls7

Other people are getting the reverse rule wrong. It would not be twice your age minus seven; it would be twice (your age minus seven).

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.

A clarification: the formula mentioned to determine WHEN an inappropriate relationship becomes appropriate is incorrect. The correct formula is to double the difference between the appropriate age of the younger person and the younger person's actual age. Conversely, this will equal the difference between the appropriate age of the older person and the older person's actual age. A SIMPLER FORMULA: The older person's age plus 14 minus twice the younger person.

Years ago this rule-of-thumb stated that a man is most attracted physically to a woman half his age plus seven. But in recent years this rule has been perverted. The new meaning is that dating a woman younger than "half your age plus seven" is considered anything from "inappropriate" to "indecent".

How superficial is that.

Other superficial rules have fallen away over time.
- Religion: In our society today, most people approve of a romance where the participants have different religions. Right?
- Height: It's ok for the woman to be taller than the man. No?
- Race/Ethnicity: They might be different races, ethnicities, or come from vasty different cultures / nations, and that is ok, right?
- Gender: A majority even approve of same-sex relationships.

But G-d forbid that a 50 year old man should date a 31 year old woman? This is indecent?

If you really are judging other peoples romantic relationships by this "half plus seven" rule, you need to do some soul searching.

There's 1300 reviews on this phrase, and no one has managed to figure out the reverse of this rule? Surely my town wasn't the brightest of the lot?

If you're 40, 27 and younger is skezy, as 40/2+7 = 27.
If you're 40, 53 and older is skezy, as 40*3/2-7 = 53.

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.