IMO the picture book range breaks down into a number of possible age ranges: As examples, 2-4 for concept books for the very young, 3-5 for simple PBs and bed-time books, 4-6 for "classic" picture books, 5-7 for PBs with school settings.... One handy trick is to look on Amazon for books similar to yours in themes and complexity of story and vocabulary, and see what age range they give there.