The formula one must use when determining at what age it is socially acceptable for a male to date/marry/etc a female.
Generally stated, the guy's age divided by two, plus seven.
Cale is 26. Cale's age divided by 2 is 13, so adding 7 to that gives you 20. Thus, by the rules stated in the Creepy Guy Formula, the absolute youngest girl Cale can date is a 20 year-old.