The lower limit on the age of a potential partner for a person over 14, by socially acceptable standards. Used as a rule of thumb.
The converse is twice (your age minus seven).
The following online calculator makes the math easy and also calculates how long until an UNacceptable relationship becomes acceptable:
Dave: Yeh man, I totally banged that Kath chick last night.
John: Are you a pedo?! She's 15!
Dave: Nah, she's 15 and a half, half-your-age-plus-seven, I'm 17, so half my age is 8.5 and plus seven is 15.5.
John: ...Doesn't change the fact that she's my sister...