I understand this - I didn't assume that the student would start at age zero. If we're going into vocational training in their teen years, they'll have to starting teaching the classics and philosophy to tweens.
Of course you can teach things about Roman Legions, or Athenian ships etc, but if you want to debate and discuss the ideas and foundations of these civilizations (and any philosophy) I honestly think you are fighting an uphill battle until at least 17-18.
Your mind is just not developed enough to handle the kind of ambiguitiy and complexity needed to analyse and judge these ideas. Many people never even reach this ability during university and just parrot ideas that are told to them.
There is a reason teaching children to adhere to a religion from a young age is so controversial in some circles.