How does Ralph experience a loss of innocence?

At the beginning of the story, Ralph crash-lands on the uninhabited tropical island with the other boys, and they attempt to establish a civil society. As the elected leader of the group, Ralph attempts to create a civilized society by enforcing rules and giving the other boys directives regarding necessary tasks that will lead to their rescue. Unfortunately, Jack and his hunters begin disobeying the rules, and Ralph gradually loses his influence over the boys. As the majority of the boys descend into savagery, the fear of the beast dramatically impacts their perspective as they look towards Jack for leadership and protection. During a tropical storm, the boys end up mistaking Simon for the beast and brutally murder him on the beach. Following Simon's murder, Ralph becomes completely dejected and laments their current state of affairs on the island.
After Jack's savages steal Piggy's glasses, Ralph and Piggy challenge Jack, and Roger ends up rolling a massive boulder towards Piggy, which strikes and kills him. Ralph not only witnesses the death of his close friend but is also forced to run for his life as Jack and his savages ruthlessly hunt him. At the end of the novel, a British naval officer prevents the boys from killing Ralph and Golding writes,

. . . Ralph wept for the end of innocence, the darkness of man’s heart, and the fall through the air of the true, wise friend called Piggy. (290)

Overall, Ralph experiences a loss of innocence by participating and witnessing the brutal deaths of Simon and Piggy. He also experiences the chaotic atmosphere of an environment void of adults, rules, and regulations. After failing to establish a civil society and witnessing each boy's primitive, savage nature, Ralph loses his childhood innocence.

Witnessing Ralph’s efforts to effect the boys' rescue, it is easy to forget that he is a child, one of the older boys, a “bigun,” but a child nonetheless. When he first realizes the absence of adults on the island, Ralph’s response reflects the immaturity of childhood. “No grownups!” he declares with delight.
As life on the island becomes more and more ominous, however, Ralph desperately wants the authority and protection of grownups. Ralph’s longing for the security he had known while living in a world of adults is reflected in the memories of home he revisits at night before falling asleep.
Ralph’s initial relationship with Piggy further emphasizes Ralph’s immaturity. At first, like the other boys, Ralph treats Piggy with disrespect and rejects him as an equal. As he matures, Ralph grows to understand and rely on Piggy. He recognizes in Piggy a good mind, more capable of reason than his own, and a courageous spirit that defies injustice.
Ralph’s maturity and its tragic implications are evidenced in the novel’s ironic conclusion. Standing on the beach under the gaze of a naval officer from the ship that has come to the boys’ rescue, Ralph’s appearance is deceiving. The officer sees before him a sobbing child with a “filthy body, matted hair, and unwiped nose,” but in Ralphs’s tears lies the reality of what he has become. He weeps not as a child but as a soul now familiar with evil, its manifestations and its horrendous consequences. He weeps “for the end of innocence, the darkness of man’s heart, and the fall through the air of the true, wise friend called Piggy.”
Ralph suffers under the weight of metaphysical truth revealed, the knowledge of “mankind’s essential illness” imposed upon him by circumstances beyond his control and by events from which he will never be free.