You'd be surprised. In Medieval Europe, kids as young as 12 were allowed to be sent into battle alongside knights as squires. Barring that, when a nation is under the very real threat of an unstoppable force coming to annihilate them, they tend to waive age requirements, such as the case with...
Yeah, I think he probably just grew up. Maybe he enlisted in what was left of the army as a way of possible following in his father's footsteps and was KIA defending Kakariko.