We have to create a line somewhere and currently that line is the age of majority. Some people mature faster than others, and even you didn’t provide a specific age. How are we supposed to make decisions about what constitutes an adult without a firm cutoff?
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.
And also can get very dead from the numerous conflicts in which the US keeps getting involved in.
[citation needed]
It's certainly the case that suicide and reckless death are more common in young people and teenagers, but I'm not convinced by all this mature brain argument as it tends to be deployed selectively to infantilize people.