Man, what a dramatic way to end the chapter.
I hate it when Ataru acts apathetically when it comes to Lum's problems and then starts blaming her for them. It's probably his worst trait.
But usually he still ends up helping her in the end.
Also, coming of age? Does that mean Lum is a fully-fledged adult now?
Edit- The anime and the original dialogue translate this as becoming an adult so yeah, that's what it really means.