Age is one piece of the decision
Parents often search for age-appropriate books because they want a simple answer. That makes sense. A child is eight, ten, twelve, or fourteen, and the parent wants to know if the book fits. The hard part is that age alone cannot carry the whole decision.
Reading level, maturity, family values, sensitivity to fear, tolerance for violence, and interest in romantic content all matter. Two children the same age can need very different answers for the same book.