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.
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.
It is often the case that cultures with either cyclical or progressive systems have equivalent ideas about cosmology and the nature of time.
Keesing (1981) gives the example of the Karimojong of Uganda, among whom around six age sets are active at any one time, with young adult men being initiated into the most junior, which is closed after fifty or sixty years, and a new one formally opened.
[1][2] The Oromo people and their Gadaa System are also another good example of a society whose social organization resolves around age sets.