“pretty girl, with faraway eyes, why do you look with such surprise? how did you get to be so wise, old girl in young-girl disguise.
what do you think this means?

I think it means that the girl seems so surprised, yet she seems like she is daydreaming and is unaware of what is truly inside her head. She has wisdom that matches that of an elder, but she is still young and learning. I think it means she looks young but she is truly smart with the knowing wisdom of an older person, and she is unaware of how great she truly is. She thinks she is wrong, and this person obviously realizes how great, and beautiful she is deep down inside. This person obviously sees her for herself.
Hope this helped