The reference here is taken from the following quote in chapter 4:
Here, invisible yet strong, was the taboo of the old life. Round the squatting child was the protection of parents and school and policemen and the law.
The “old life” refers to the boys’ lives prior to crashing on the island and having to fight for survival. Their new life on the island is one where there are no adults. By implication, there is no one to ensure the boys’ behavior fits within the boundaries of moral and societal expectations. In this description, Roger observes a younger boy, Henry, and starts to test the boundaries which he, Roger, had been taught while growing up within the “norms” of society.
In all aspects of the “old life”—at school, at home, in public—Roger would have been taught not to be maliciously violent and to not bully younger boys. Here he is tempted to test these boundaries by throwing stones in Henry’s direction. However, the adult representatives of rules and orderly behavior—“parents and school and policemen and the law”—are still uppermost in Roger’s mind. The memory of these adults ensures that Roger’s behavior does not overstep the boundaries in that he does not make a direct hit on Henry with a stone.
The “old life” is thus the reference to all the rules and guidelines for behavior which a young boy would be taught at home, at school, and in society. In the same way certain things would be taught to be "taboo" or things which should never be done. One such taboo would be to purposefully throw a stone at a younger child with the intention of harming this child. This is what tempts Roger and he tests the boundaries in this extract.