Innocence is almost an enemy in All Quiet on the Western Front – a soldier must get rid of it right away if he wants to survive. Our narrator learns to lose his innocence quickly, mostly by observing that loss in others around him, but also through his own experiences. And he consciously feels that loss as he survives in the story, noting how he has become "old folk" and how separate he feels from the new recruits entering the fray, recruits only a year or so younger than he is, but far more innocent.
Because he has become so numb, Paul regains his innocence at the very end of the novel.
The soldiers never lose their innocence.