"But then, shall I never get any older than I am now? That'll be a comfort, one way -- never to be an old woman." This reflects a whimsical wish to stay forever young, which is a common fantasy about avoiding aging and its limitations.
"But then -- always to have lessons to learn!" emphasizes the idea that remaining young also means continuously facing new challenges and learning experiences, suggesting that growth and change are inevitable parts of life regardless of age.