Under the Act on the Promotion of the Use of Bicycles, guardians of children under ( ) years of age must not let them ride an electric bicycle. Which number fits the blank?

Answer: ② 13

Guardians of children under 13 must not allow them to ride an electric bicycle. Electric bicycles are faster and heavier than ordinary bicycles, so young children are kept off them. [Act on the Promotion of the Use of Bicycles, Art. 22-2]