how does the boys age influence their savagery and how would the circumstances and action be different if the boy were younger or older?

They do have "littluns" on the island that are around five years old. They live a life of isolation away from the "bigguns". I think boys that were any older than Ralph or Jack might have upset Golding's social experiment. Older boys might have been too invested in the adult world to demonstrate the choices boys make when left to their own devices.