 The "Spartiate" population was subdivided into age groups.The youngest at 20 were counted as weaker due to lack of experience, and the oldest, up to 60 or in a crisis 65, were only called up in an emergency, to defend the baggage train.The principal source for the organization of the Spartan Army is Xenophon, who admired the Spartans and whose "Constitution of Sparta" offers a detailed overview of the Spartan state and society at the beginning of the 4th century BC.