Young boys were chosen because although they have been civilized somewhat, but still require adult supervision. If adults were abandoned on an island, you would not expect their civilization to deteriorate as quickly or as completely as the boys’ did.
At first, there is some semblance of society, but the children are just copying what they know of adult social order.
The children gave him the same simple obedience that they had given to the men with megaphones. (ch 1)
The older boys basically pay little attention to the younger ones, and no one but Piggy ever tries to even learn their names.
Because they are children, they do not know how to create a society and have to create one from scratch. Golding uses this to establish the base of humanity. Consider the kids’ first reaction to hunting, when Jack doesn't cut the pig.
They knew very well why he hadn’t: because of the enormity of the knife descending and cutting into living flesh; because of the unbearable blood. (ch 1)
Later, they have no problem killing Piggy when they think he is the beast. This demonstrates how quickly the devolve into chaos.