By certain human rights orgainization we do.
Since a 17 year old can join with parents concent, some human rights organizations say the US does sponsor child soldiers. However, these "child soldiers" do not get deployed until after they are 18. The vast majority do not even complete training before they turn 18. Those organizations don't tell you that. Also, the majority are split option meaning they take Basic Training their Junior year in High School and then finish technical training the summer of their Senior year, so they are usually 18 by then as well.
I could see it possibly happening if something drastic happened, like 50% of the population of the contry died from a plague or something all at once, during a time of war, but under normal situation, no.
The other thing to consider with child soldiers is that the countries that have them, the age of adulthood is lower. The idea of 18 being an adult is true in the "western world" mindset, they don't consider that in a lot of "non-western" cultures 16, 14, even 12 is considered an adult and therfore could be a soldier. In most of those cases its a matter of culture, so you have to examine this argument too, is it right to force a "western" culture idea on "non-western" cultures.
Even in our own "western" culture the idea of 18 being an adult is only fairly recent in conception, in the 1800 and before our age of concent was a lot lower and we had child soldiers in federal service preforming limited roles, such as drummers, couries, strecher bearers, and ammunition bearers. So it has already happened.
If you look at cases like Ruby Ridge and the Branch Dividians, extremist against the US government had children in their compounds and even train them. So the idea is out there, but I do not thing there would any type of popular uprising like you see in Africa.