The narrator of The Catcher in the Rye is an adolescent obsessed with saving children from the dirtiness he sees in the adult world. The novel deals with innocence in many forms, but focuses often on the sexual. Because the narrator sees sex in any form as dirty, he feels the need to sequester children (and himself, somewhat) from it, instead of easing into it as a natural step to becoming an adult.
Although Holden is obsessed with topics like sex, he betrays a childlike innocence in the way he looks at the world.
Holden is actually wise beyond his years; this makes his desire to protect the ignorance of youth an ironic one.