Yeah I looked it up and it stems from the French of all people, who you would have thought would have been perfectly fine with young love, but there we go. Maybe It comes from the puritan movement?

Attributed to French author Max O'Rell in his 1901 “romantic guidebook” with the wildly unsexy title, Her Royal Highness Woman and His Majesty Cupid, the mathematical formula was O'Rell's unofficial law re: romantic age gaps. According to his calculations, a bride's ideal age was half the groom's age plus seven years.

So I think we can retire that one. Also notice the use of the word ideal age. It wasn't a mathematical formula for calculating the youngest that it's acceptable to date. It's a mathematical formula to calculate the ideal age to date, presumably there is a lower lower bound.