There are lots of terms that describe a social construct that exists as a concept in some cultures, but that has no real equivalent in other cultures. By this, I mean that the term groups other people in a manner that just wouldn't seem obvious or natural as a category to some cultures, but that have their own sort of logic.
For example: would it seem weird to you to use the same term for and only for people your age, people your grandparents' age, and people your grandchild's age? And to use a separate set of terms for people your parents' age and people your child's age?
Well, some cultures have a system of alternating generations. If we number generations, for a generation n, we can do n mod 2. If this is equal to 0, then they're in the first group; if it's 1, they're in the second group.