Yes, that's a really good point, and something that is taken very seriously because of the UN Convention on the Rights of the Child and its protocol on child soldiers.
When I talked about the identification process, I can tell you from my experience that where something like age is in doubt, a doctor is retained to come in. A lot of times, someone won't know how old they are. A father might even say, “Here's my son. He's of the right age.” That's said to keep them together when they go to a recruitment place; the age span of some of the recruits would impress you.
There are ways of ascertaining age with a reasonable degree of certainty, and where recruits' ages have been determined to be within the range of a child, their training is discontinued and they are sent back. Sometimes that puts a strain on the dynamic in that group, because in some cases you'll have two or three or four from a village who are coming up to join the army or the police.