Oooh, that's an interesting question. So, normally I'd reject the hypothetical here, and say war is bad, so "should" doesn't apply. But instead, if I can assume it's required for existence, society should define the minimum age as high as possible to ensure victory. Fighting in a war is actively harmful to feeling safe. Feeling safe is required for individuals to grow at the maximum rate. Growing as a child has the same effects as compounding interest, so the absolute best growth possible should be the target for everyone, as early as possible.

If you're talking about joining a standing army without consideration for combat. I don't think there's any minimum age. I used to teach a "combat sport" to children as young as 4. Then, for the US army, which I know about, I think 18 is perfectly reasonable, where many people could gain very useful skills/education from the training that joining the US military entails.