How do we know that lizabeth is on the verge of becoming an adult? marygolds book?

Going back a bit in the story she heard her father crying because he couldn't provide for the family and this made her realised that adults were also vulnerable.

It also made her angry and this is why she destroyed Miss Lottie's flowers

When she finished she saw Miss Lottie looked up and saw Miss Lottie and how devastated she was that her flowers had been destroyed

She linked her father's crying and Miss lottie's upset and suddenly feels sorry for Miss Lottie and has started looking at things with an adult rather than a childish/teenage view.