Athenian Ephebate Question

I'm trying to make sense of the Athenian ephebate. Some of you may have come across more definite answers to the question posed below, so I submit to your expertise:

I'm trying to learn more about the Athenian ephebate of the 5th century. A lot of superficial books on the subject (we'll call these of the Osprey sort) claim that the ephebate was up and running during the Peloponnesian War. If the ephebate did exist, it certainly didn't exist like it did during the later 4th century where the emphasis shifted from military training to "cultural training." However, I've recently come across some papers that claim that the ephebate was, itself, an invention of the 4th century.

When the ephebate did exist, Athenian citizens were trained from ages 18-20, during which they served garrison duties in Attic forts. According German texts on the Constitution of Athens, it was illegal for someone under the age of 20 to serve outside of Attica.

However, in Plutarch we know that Alcibiades fought at Potidaea before his 20th birthday. So, either the rule prohibiting service abroad had not yet been imposed or the Ephebate did not exist like it did during the 4th century (and when Aristotle wrote his treatise on the Constitution of Athens).

Finally, if the Ephebate did not exist during the Peloponnesian War, how - if at all - were citizen hoplites in Athens trained.

Thanks for any help!