I find it also interesting that Lord of the Flies was also written as an allegory. These children are symbolic of society. This situation demonstrates what would occur if peoples were misplaced and all of the sudden someone had to become the leader of a group. Piggy is certainly missing adults, he is one who seeks boundaries and longs for an adult to provide it. Our society today would experience similar flaws if a number of us remained after an atom bomb and had to start over without our original leaders and civilized processes in place. Piggy's reference to "the majesty of adult life" mirrors those things in this world that the common person does not know because they rely on legislators, teachers, doctors and lawyers to know it for them. I don't think the boys are failing because they are not grown-up, I think it has to do with being thrust into a new situation and not knowing how to deal with it. Part of that will come with age of course, but several of the boys should have some survival skills by this point in life.
In many ways Piggy's quote is ironic. The problem that led to the boys being in the aircraft that was moving the boy's to safety was caused by the adults not being able to orchestrate a peaceful life. As a result the world that Piggy and Ralph had come from was at war and the boys were being flown to safety when they landed on the island.
The things that Piggy is considering is that adults have the knowledge of how to prepare and organize civilization and maintain it which is questionable. However, from a child's point-of-view the adults seem to be the ones with all of the answers. Piggy seems to believe that adult knowledge is a powerful thing which could keep them safe and protected.
The boys are not failing because they are not grown up, but rather scared at the unfamiliar situation.