The minimum age to join a PADI Jr Scuba Diver or PADI Jr Open Water Diver beginner diving course is 10 years. Younger children (8+) can participate in programs that do not yet lead to a certificate, but they can gain their first experiences which will help them in their further diving education. Such programs are PADI Bubblemaker and Padi Skin Diver.
Different courses have different age restrictions, which can be found in the description of each course.