What does the narrator say about Félicité's age and appearance and why might this be an important detail?

The narrator tells us that when Félicité was younger, she appeared to be older, and that her age became a complete mystery as she aged. That she looked old when she was young suggests an inherent wisdom. As she grows older, she become ageless, which suggests that there might be something eternal about her character. Perhaps her character isn't formed by life lessons, as we generally think, but on something intrinsic to her nature.