Age cohort
People born in a given 5-year interval form a cohort which moves through time together. Mostly a convenience for demographers (who like to deal with comparable groups), the general concept of age grades has wider applicability and considerable subtlety in specific cases. See a Web page on the Bunun, an 'aboriginal' people of Taiwan, of whom it is said that "...the age cohort to
which a person belongs determines their lifestyle and treatment by
society". The same might be said of American society, don't you think? I've always meant to write an article on "Age grading among the Nacirema"...