Senufo-Tagba of West Africa
An age set is a group of people born within a couple of years of each other. Age sets serve as a hierarchy of knowledge and service within Senufo culture: a person always knows who is older and who is younger. The person gains knowledge from an older person and passes that knowledge to a younger person. When a man is asked how old he is, he will usually find someone in his age set and reply, "I am the same age as he is."