Battle of Mantinea
Epaminondas began serving as a soldier after adolescence; Plutarch refers to an incident involving Epaminondas that occurred during a battle at Mantinea. Though not explicitly stated, this was probably the Spartan attack on Mantinea in 385 BC, as described by Xenophon; Plutarch tells us that Epaminondas was there as part of a Theban force aiding the Spartans, so this battle fits the description. Epaminondas was certainly not old enough to have served at the First Battle of Mantinea which was in 418 BC.