My two oldest daughters were both certified when they were 12 and 10 (they are now 14 and 12)
I started them out from birth in a pool and they were swimming at age 4 to 5.
After they could swim I started them on a snorkel and mask in the shallow end then to the deep end diving down and swimming to the shallow. By age 6 let them start breathing on my octo while swimming them around under my arm. Around age 8 started letting them wear the gear themselves and use it in the shallow end, all the time I am instilling the importance of safety, and i am always right beside them at all times, by age 9 they both could set all the gear up and do a safety check, do all basic underwater skills and even clear their mask. At age 9 I also took them through the open water diving book and taught them how to do dive tables.
So by age 10 (with my oldest being 12) they both went through OW class with no problems and were both top of their class in dive skills and scored a 98 and a 96 on the final exam.
All that being said I spend as much time with my kids as I possibly can I love being with them, and believe that if your kids are interested in something like diving at an early age then a parent must spend an enormous amount of time with them.
At 14 and 12 I do not allow them to go out on a boat dive into the open ocean, They dive a quarry that is 25' max depth (perfect for them to train in and get experience) and when we go to the beach we will dive the jetties. they both have about 50 dives in the quarry and 10 or so on the jetties.
On a final note I did not push or even suggest them getting into diving they both decided on their on at a young age.
And i also have two more daughters ages 4 and 2 who are already showing interest also.