Why does Lily say " I was thinking how much older fourteen has made me. In the past space of a few hours I'd become forty years old?"

This quote comes directly after the section in the novel where Lily realizes that maybe her mother's death hadn't been her fault.... maybe T-Ray had just decided to punish her for it. This realization causes Lily to pack her things and head out..... it made her feel like she'd aged and become an adult at the same time.