@Guytsu Well, like I said, since it's an unwinnable battle (and moronic), I never paid much attention. I'm not sure how "serious" the formula is, but it is very much a thing for those people.
It's not "7 year gap" or "10 year gap". It's about adding idk what numbers and dividing for 2 or some shit.
Let me put it this way. If your parents are 35 and 45, you don't see much problem, right. But if the couple is 15 and 25, you may have a bit of an issue, whereas you wouldn't if they were 15 and 17.
It's something along the lines of "the older you are, the bigger the age gap can be".
But not sure of the details.
EDIT: Decided to google it.
Dating Age Rule. The old rule of determining a socially-acceptable age difference in partners goes something like this: half your age plus seven (40 = 20 +7 = 27) to define the minimum age of a partner and your age minus seven times two (40 = 33 * 2 = 60) to define the maximum age of a partner.