It is for a good reason that the official age of recruitment is usually very young:
16 for the British army,
17 for the American, Bahrain, Iran, Kazakhstan, Mexico, Pakistan, Tajikistan,
And Turkmenistan will make soldiers of 15 years old children.
The reason being, most likely, that at this tender age the concept of Death is unfathomable, and the poor youngsters feel invincible.
Till they’re killed, and then it’s too late.