Age gap calculator
An age gap is simply the difference between two birth dates. The half-your-age-plus-seven rule of thumb says the youngest socially acceptable partner is half your age plus seven; at 40 that is 27, and the oldest is 66.
How to use the age gap calculator
The rule has no research behind it and appears in print as early as the nineteenth century as social advice rather than a finding. It has one interesting property: it is self-consistent, meaning if A is acceptable to B then B is acceptable to A, which is not true of most rules of thumb people invent. Below about age fourteen it produces a range narrower than a person, which is where its usefulness clearly ends.
Questions
Divide your age by two and add seven for the youngest; reverse it for the oldest.
None. It is a piece of social folklore that predates any relevant research.
Yes, unusually, if A is within B range then B is within A range.
Below about fourteen it produces a range narrower than a single year.
No. The difference between two birth dates is fixed.