The minimum age to get your SCUBA license is 10. I advise that kids close to this age be strong swimmers and mature enough to study their training materials with minimal help.
Any class with a 10 or 11 year old in it is limited to a maximum ratio of 2 students to 1 instructor. When the child turns 15, you can convert their license from Jr. OW to Open Water. Divers aged 10 to 14 must dive with an adult certified diver at all times.