“I was on the trail of something that would lead me out of childhood”. What does David mean by this statement and why is it important to the text as a whole?

David is alluding to the fact that his childhood was about to end... to be taken away by the actions of his uncle, who had raped Marie... the woman who cared for him. This is important to the text because it marks David's coming of age.

Had I any sensitivity at all I might have recognized that all this talk about wind and dirt and mountains and childhood was my mother’s way of saying she wanted a few moments of purity, a temporary escape from the sordid drama that was playing itself out in her own house. But I was on the trail of something that would lead me out of childhood.