In its first known appearances (in the book Her Royal Highness Woman by Max O'Rell, "The little shepherd of Kingdom Come" by John Fox, and the 1951 play The Moon is Blue by F.Hugh Herbert), it's stated or implied that at the beginning of a relationship or marriage, the woman's age "is supposed to be half the man's age, plus seven", which gives a different (asymmetrical) interpretation to the rule.
Graph of the Half-age-plus-seven rule ("never date anyone under half your age plus 7"), which claims to dictate what age disparity between two people is acceptable in dating/romantic/intimate relationships during the late 20th century / early 21st century (called the "Standard creepiness rule" in the xkcd webcomic).
According to this rule, the age of the younger person should not be less than half the age of the older person plus seven years, so that (for example) no one older than 65 should be in a relationship with anyone younger than 39 and a half, no one older than 22 should be in a relationship with anyone younger than 18, and no one under 14 years of age should be in a relationship at all...
For convenience, the line representing x=y is also shown (in blue).
The area between the blue and red lines shows where you are the older partner in the half-age-plus-seven calculation, while the area between the black and blue lines shows where you are the younger partner.
The rule, whose origins remain mysterious, has been passed down through generations as a way of justifying or, more commonly, pouring scorn on other people’s couplings.