In January 2015 the UN confirmed that the Burmese Army had, in 2014, discharged over 400 soldiers who were younger than 18. This was in compliance with the UN Child Soldier Protocol, which most nations have signed but not all have complied with. The UN Child Soldier Protocol bans the use of soldiers younger than 18, and only allows enlistment of soldiers under 18 with the permission of their parents.
These protocols have become more restrictive since the 1980s. At first the protocols banned the use of children under age 15 in combat. In much of the world it is common to use kids under age 14 in the armed forces (regular or irregular). In the West this was the case until the 19th century and by the 20th century the Western custom was to take no one under 17. Thus before 1990s it was common to find 17 year old American soldiers, marines and sailors in combat zones.
There were also many cases of underage boys who enlisted using false identification, sometimes with the knowing consent of their parents. This was fairly common until the 1960s. The total number of underage American soldiers who died in Vietnam is unknown, but at least five of the U.S. troops killed in Vietnam are known to have been under 18 years of age. One, a Marine, seems to have been only 13. There were even more cases during World War II. If found out, such young soldiers were usually removed from the combat zone (especially if under age 18).
As it turned out setting 18 as the age one magically becomes an adult proved to be a contentious issue when the current Child Soldier Protocol was being drafted. Many nations had, for centuries, taken in recruits who were 17 or younger. Most of these nations agreed that, for all practical purposes, there was no difference between 17 and 18 year olds. But the UN bureaucrats had their way, and 18 became the age at which you were no longer a "child soldier."
Nevertheless, it's still popular for 17 years olds (usually recent high school graduates), to enlist in the military. Nearly 10,000 17 year olds do so each year (most for the army) in the United States. It was only two years ago that the U.S. Marine Corps, reluctantly, stopped sending 17 year olds into combat. These were usually 17 year olds only a month or two short of turning 18. They had already finished six months of very tough training, only be told they were "too young" to go off to combat with the guys they trained with. This hurt morale in the entire unit.
In Africa and the Middle East it has become common to use children as young as nine or ten as suicide bombers (often without telling the kids what was going to happen with that heavy vest they were wearing when they go to where they were supposed to go). More commonly kids this age are used as lookouts and couriers, as kids this age have been used for centuries.