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.

> most people could gain very useful skills/education from the training that joining the US military entails

And also can get very dead from the numerous conflicts in which the US keeps getting involved in.

Even during peaceful years like the 90s or in non direct combat roles being in the military is very hard on the body though infantry get the brunt of it.