The Magic Pipe, livre audio

According to the legend, The Pied Piper lured children to their graves with his evil, enchanting flute music after town leaders refused to pay him for ridding Hamelin of rats.
One question: When did the children of Hamelin stop being children? At what age did children become immune to his tune? Twelve? Sixteen? Eighteen? Or was it a more gradual process?
This short story is a dark re-visioning, interrogating an age old question: What does it mean to be considered 'adult'?