Golding most likely chose preteen boys to show a more rapid decline into savagery. Older children, or adults even, would be more conditioned to act a certain way, more inclined to mimic the laws and civilized behavior of their past.
Golding's children have been taught by their parents, but they are very immature. They see the island as fun and games. Ralph at first does not take his leadership seriously. Through the boys' experience on the island, Golding shows their rapid devolvement into savagery as well as Ralph's painful loss of innocence and growing maturity.
There are roughly three age groups. We have the older ones who are close to the age twelve: Ralph, Jack, and most likely Piggy. We have the middle ones, who are around ten: the twins, Roger, Robert, and Simon. And then we have the littleuns, who are six and under:
The smaller boys were known know by the generic title of "littluns." The decrease in size, from Ralph down, was gradual; and though there was a dubious region inhabited by Simon and Robert and Maurice, nevertheless no one had any difficulty in recognizing biguns at one end and littluns at the other.
As typical of neighborhood groups of children, the oldest and the largest becomes the leader. In this case, Ralph fits this description. We are not told Jack's age, but Ralph we do know is broader than Jack.
Since these characters are not quite teenagers, I'm not sure if Golding is making a point about teenagers in general, but I do think he is portraying quite aptly the dynamics of group interaction. We have the leader--Ralph. We have organizers and confidantes: Piggy and Simon, and we a challenger, aggressor, or trouble-maker: Jack. In groups we often have those who are uninvolved or apathetic in the decision-making. In this case, these are the littluns. Golding's characters reflect the roles of most social groups whether it be a school club, a classroom, a church congregation, or town.
The various tiers of ages also show the eternal power struggle. The power struggle between Jack and Ralph will be echoed in the middle children and later in the younger ones. In other words, Jack is not the sole cause of conflict on the island. Behind Jack is Roger, waiting for his chance to be the aggressor. The littluns, it is suggested, will have a similar dynamic. In this way, Golding shows us that the enemy is not something that we can hunt and kill, that it is within us. The conflicts that the children face are those that all groups face: power struggles, apathy, conflicting priorities, lack of teamwork, and dissension.