Per Wikipedia:
One rule of thumb to determine whether an age difference is socially acceptable holds that a person should never date someone whose age is less than half their own plus seven years.[80][81][82][83] According to this rule, a 28-year-old would date no one younger than 21 (half of 28, plus 7) and a 50-year-old would date no one younger than 32 (half of 50, plus 7).
Although the provenance of the rule is unclear, it is sometimes said to have originated in France.[81] The rule appears in John Fox Jr.'s 1903 novel The Little Shepherd of Kingdom Come,[84] in American newspapers in 1931 attributed to Maurice Chevalier,[85] and in The Autobiography of Malcolm X, attributed to Elijah Muhammad.[86]
In many early sources, the rule was primarily presented as a formula to calculate the ideal age of a female partner at the beginning of a heterosexual relationship. Frederick Locker-Lampson's Patchwork from 1879 states the opinion "A wife should be half the age of her husband with seven years added."[87] Max O'Rell's Her Royal Highness Woman from 1901 gives the rule in the format "A man should marry a woman half his age, plus seven."[88] A similar interpretation is also present in the 1951 play The Moon Is Blue by F. Hugh Herbert: "Haven't you ever heard that the girl is supposed to be half the man's age, plus seven?"[89] Despite this, there are contemporary sources indicating that a woman falling below this target age was still considered inappropriate, or otherwise a hindrance to the relationship. For example, in John Fox Jr.'s aforementioned 1903 novel The Little Shepherd of Kingdom Come, the rule is cited immediately before a woman is described as being "too young [for her potential partner], and she can wait."[84]
A 2000 study found that the rule was fairly accurate at describing the minimum age of a woman that a man would marry or date. However, the rule was not found to be descriptive of the minimum age of a man that a woman would marry or date, nor (by reversing the formula) of the maximum age that either sex would marry or date.[80]
This rule implies that the younger individual should be at least 14 years older than the difference between the two ages[90] and the older individual should be at least 14 years older than twice their age difference.[91] For example, a couple with a 10+1?2-year age gap must be aged at least 24+1?2 and 35, respectively, for the rule to be met.
