The Trojan War started because, at the marriage of Thetis and Peleus (Achilles's parents), Eris, goddess of discord, shows up with the Golden Apple of Discord, inscribed "to the fairest." Since Hera, Athena, and Aphrodite each want the apple, this leads to the Judgement of Paris, where Paris, prince of Troy, decides that Aphrodite is the fairest because she offers him the most beautiful woman in the world as a reward. Helen is already married (with, I think, a nine year old daughter, Hermione) to Menelaus, but she takes off for Troy with Paris anyway, thus setting off the Trojan War. So, even given some time for Paris to go find Helen, then some more time for the Greeks to try to get Helen back without a war, I still don't see how Thetis and Peleus's son Achilles could possibly have been old enough to fight, much less to have a son of fighting age along at Troy with him.
I have Stephen Mitchell's Iliad, which Travis is reading while he listens to the audio book (the Mitchell translation has a 16 hour audio book, which seemed more Travis-friendly to me than the twenty-two hour audio book for the Lattimore translation), and I have Richard Lattimore's Iliad. I also have Malcolm Willcock's A Companion to the Iliad, and I am listening a second time to Elizabeth Vandiver's Iliad lectures. And I am working through the Harvard edX course, The Ancient Greek Hero. And would you believe that not One of them addresses the issue of how Achilles can be the right age to fight in the war, given that the war started with a fight at his parents' wedding? I don't think that Paris was the kind of fellow to wait twenty-something years to go visit Helen. Of course, I know you will say, "Bernard Knox addressed this problem fully in his introduction to Fagles' Iliad!" But you are just saying that to tease. Right?
I brought up the question on a homeschool group on Facebook, and we actually Did solve it. Only, the solution requires the Tardis ex Machina, which is only satisfactory if you think the Greeks were so familiar with the Doctor that this seemed so obvious to them as to require no explanation. I'm not sure.