The Rwa farm the densely populated slopes of Mount Meru, facing Mount Kilimanjaro. Their age system is part of a highly developped sociopolitical organisation, with a centralised chieftaincy and powerful patrilinear clans. We will first describe the logic and structure of this age system, and compare it to those of the neighboring Maasai and Arusha. This age system fulfills various functions, implying cognitive, matrimonial, demographical, social, legal and ritual aspects, which will then be described. Thirdly and lastly, we will address the history and reasons for the evolution of the Rwa age-system. From a leading role at the end of the 19th century to a near collapse in the 1960's, it now meets with a powerful comeback but is entirely subordinated to the chieftaincy. Owing to the Rwa's desire to assert their identity in a changing world, the age system is currently thriving and gives rise to big ceremonies.