How we cite our quotes:
"I'll wash to-night. […] Lemme be, I tell you." (25)
This is what Sarty says when his mother tries to wipe the dried blood from his forehead after he's knocked down by the other kid. This moment helps us feel how young he is, even though we don't know his exact age yet.
In the early afternoon the wagon stopped before a […] two room house identical almost with the dozen of others it had stopped before […] in the boy's ten years. (32)
This is how we know Sarty is ten, so this moment is important. It also shows us how monotonous, predictable, and limited his life has been, even with all the adventure of being Abner's son. At ten years old he is locked into a perpetual cycle of coming and going, but always with regard to the same situation.
[…] the terrible handicap of being young, the light weight of his few years, just heavy enough to prevent him soaring free of the world […] but not heavy enough to keep him footed solid in it […]. (40)
In other words, Sarty is just beginning to understand the way the world works. As such, he doesn't feel completely lost in the chaos of his father's world. On the other hand he lacks weight. As far as the world is concerned he has no authority to determine his own life.