Xenophon, Cyropaedia
Walter Miller, Ed.

[13] And these, in turn, for twenty-five years¹ after the time they are there enrolled, are occupied as follows. In the first place, like the youths, they are at the disposal of the authorities, if they are needed in the interest of the commonwealth in any service that requires men who have already attained discretion and are still strong in body. But if it is necessary to make a military expedition anywhere, those who have been thus educated take the field, no longer with bow and arrows, nor yet with spears, but with what are termed “weapons for close conflict”—a corselet about their breast, a round shield upon their left arm (such as Persians are represented with in art), and in their right hands a sabre or bill. From this division also all the magistrates are selected, except the teachers of the boys.

And when they have completed the five-and-twenty years, they are, as one would expect, somewhat more than fifty years of age; and then they come out and take their places among those who really are, as they are called, the “elders.”