Xenophon Cyropaedia 1.2.15

[15]
  Now, that the whole constitutional policy ¹ of the Persians may be more clearly set forth, I will go back a little; for now, in the light of what has already been said, it can be given in a very few words. It is said that the Persians number about one hundred and twenty thousand men ² ; and no one of these is by law excluded from holding offices and positions of honour, but all the Persians may send their children to the common schools of justice. Still, only those do send them who are in a position to maintain their children without work; and those who are not so situated do not. And only to such as are educated by the public ³ teachers is it permitted to pass their young manhood in the class of the youths, while to those who have not completed this course of training it is not so permitted. And only to such among the youths as complete the course required by law is it permitted to join the class of mature men and to fill offices and places of distinction, while those who do not finish their course among the young men are not promoted to the class of the mature men. And again, those who finish their course among the mature men without blame become members of the class of elders. So, we see, the elders are made up to those who have enjoyed all honour and distinction. This is the policy by the observance of which they think that their citizens may become the best.