Children pay reduced prices for tickets on trains and at various tourist spots in China. The method is simple, as shown in the photo here. Children whose heights are less than 1.2 meters go free. Those who are taller, but below 1.5 meters pay half the adult price.
India has rules which depend on the age of the child. I’ve sometimes seen long arguments between an official and parents about the age of the child. The official rules one way, the parents disagree, but do not have the documents to prove their point. An argument ensues, which can only be won by the official.
In China the rule is simple: easy to check and enforce. The Chinese are practical!