How long has he been a soldier? For what were Ishmael and fourteen other boys chosen? Where are they taken, and by whom?

In Chapter 15? Ishmael is fifteen years old.

A truck enters the village, discharging four men in UNICEF t-shirts. The boys are lined up and Lieutenant Jabati chooses fifteen of them, seemingly at random. Beah and his friend Alhaji are chosen, while Kanei is not. The chosen boys are ordered to turn in their weapons and equipment; Beah secretly disobeys the order by hiding his bayonet and a grenade in his clothing, then refusing to undergo a search by another soldier. The boys are loaded into the truck with several Military Police (MPs) - “city soldiers” whom Beah despises for their clean uniforms and weapons that belie no combat experience. Beah becomes increasingly anxious and angry at the seeming betrayal of his commanding officer. After several hours, the boys are unloaded in Sierra Leone’s capital city, Freetown.