300 is, by and large, a fantasy and this is one of the fantasy elements, but when I watched it, it bothered me because Paul Cartledge's The Spartans was fresh in my mind and he had said that Cleomenes I (Leonidas' predecessor) probably did not experience the agoge because he was the heir apparent to one of the two Spartan hereditary thrones. Cartledge also said that the marital arrangement between his daughter Gorgo and Leonidas was probably arranged because Leonidas had a reasonable claim to the throne. That is, Leonidas did not become king at age 10 and did not win his right to the throne by besting a beast, but by marrying the king's daughter. (Sparta wasn't Egypt, men didn't marry until mature.)
Is it true that Cleomenes I probably would not have gone through the agoge? A forum poster was having trouble finding source material that backed up this bit of "common knowledge." Paul Cartledge came to the rescue. Read: Who Said the Spartan Heirs Apparent Did Not Undergo the Rigorous Agoge Training?