The United States has many branches; Navy, Air Force, and Coast Guard. For military service you need to be at least 18 yrs. old, and at 17 with parental consent. Manpower fit for service is 59,413,358(males only).
Comparing the two the age for military service is pretty much the same. The United States has more military branches than Sierra Leone. Military size of people fit for war is much larger in the US.
References: CIA The World Fact Book, "Sierra Leone", October 20th, 2008.