I just read Rex Warner's translation of the Anabasis, and I was blown away by Xenophon's story and fascinated by Xenophon as a character. He strikes me as a very daring soldier, and a great leader of men (not totally surprising that he comes off pretty well, given that he did write the book himself). But Xenophon's youthfulness really comes across in the book as well: There's the fact that he feels the need to consult with his teacher Socrates before going on the expedition, or the way he almost cries when he discovers that all the Greek generals have been assassinated.
So I'm curious, how old was he during the events of the Anabasis?
It is not exactly known when Xenophon son of Gryllos was born. However, he suggests in the Anabasis that he was slightly younger than his friend Proxenos, who was about 30 when he was murdered by the agents of Tissaphernes (2.6.20). It is therefore assumed that Xenophon was not yet 30 at the time of the expedition in 401/0 BC. In his standard work Xenophon (1974), J.K. Anderson assumed he was born "a little after 430 BC". In his commentary on the first part of the Hellenika (1989), Peter Krentz argued that he must have been born somewhere between 430 and 425 BC. This means he was in his late twenties when he took part in the March of the Ten Thousand. To the Greeks, this meant he was still young - too young to qualify for the office of councillor or general at Athens.