Which characters lose their innocence in The Outsiders?

In The Outsiders, the main protagonists are Ponyboy Curtis and Johnny Cade. These are the characters whose loss of innocence drives the narrative.

Ponyboy is feeling alienated and distanced from his older brothers Sodapop and Darry. Knowing he isn't meeting Darry's expectations makes him feel distanced from his brothers as he struggles with his position straddling childhood and adulthood.

Johnny Cade comes from an abusive home—which in its way has robbed him of a great deal of innocence. Though he's two years older than Ponyboy, he is small and sensitive, holding onto his vulnerabilities in ways the older boys have long since let go off.

The other Greasers (the older Curtis boys, Two-Bit, and Dallas) have long since shed their innocence. They've transitioned fully to adulthood and the harsh realities of poverty, and a lack of parental guidance has forced them to survive through any means necessary. Dallas can be considered the least innocent among the characters—as he has spent time in prison and has hardened himself in response to things he has lived through and seen.

Dallas acts as a mentor to Ponyboy and Johnny when they're on the run after Bob's murder. Dallas has more first-hand knowledge of the justice system and ways to evade it. He also has a soft spot for the younger boys and doesn't want to see Ponyboy and Johnny face the same consequences he did.

This is the significance of Ponyboy's repetition of Robert Frost's "Nothing Gold Can Stay." In The Outsiders, what is "gold" is innocence and the freedom of unobstructed youth. Nothing gold can stay because nothing truly pure and innocent remains so forever. It becomes especially poignant when Johnny dies; he fought against the loss of his sensitive, openly vulnerable spirit in a hard, uncaring world, but ultimately, he lost his life.