While a year group or class in a school could be regarded as a simple example of an age set (e.g. 'Class of 2004'), the term is most commonly used to refer to systems in tribal societies. The phenomenon is most prevalent in East Africa, central Brazil and parts of New Guinea, where in many societies the importance of social groupings based on age eclipses that of social groupings based on kinship and descent. Age sets in these societies are formed by the periodic grouping together of young people—usually men—into a corporate unit with a name and a collective identity. As its members age the set stays together and increases in seniority as older sets die off and new ones are formed beneath it.
Age sets and the systems within which they exist can be regarded as either cyclical or progressive. In a cyclical system there is a finite number of sets and each recurs over the course of a few generations, with new membership. In progressive systems an age set appears once, and when its members have died it ceases to exist. It is often the case that cultures with either cyclical or progressive systems have equivalent ideas about cosmology and the nature of time.
When most members of a generation set have died off, its surviving age sets are retired and the junior generation set becomes senior. At this point, new initiates become the first members of the next generation set in the sequence. The senior generation set is responsible for initiating new members into the most junior age set of the junior generation set, and each age set is formally subordinate to the one above it. While members of an age set live with their immediate families and local kin groups, and age sets are not tightly organised internally, they serve to apportion roles and status in wider social situations, with senior age sets having a judicial function, for example.