Why do you think Golding creates two groups of boys: "biguns" and "littleuns"? Why not have all the boys the same age?
Whenever a writer has a story in which all the characters are the same in almost all respects there is a big problem in making it possible for the reader to tell them apart. In Golding's novel he has a whole group of characters who are all boys, who are all students in the same private English school, who are all even wearing the same school uniform when the story begins, who are all marooned. One way he could differentiate some of them was by age. By making some of them "littleuns," he was able to avoid having to spend much time characterizing them. He only had three characters to focus on: Ralph, Piggy, and Jack. Piggy was different because he looked much different from all the others. Ralph and Jack had contrasting personalities. Even though there are lots of boys on the island, we tend to remember just those three characters. These boys are not real people but just names on paper. It is a struggle for any author to make characters believable, and the more characters involved, and the more the characters are alike, the more difficulties the author faces. Typically, an author will "orchestrate" his characters by having males and females of different ages, different social classes, and sometimes different races. Golding was challenged because he wanted to tell the kind of story he did.