I know that my opinion on this topic has not many supporters. But this is the experience of me, my wife and our two sons (who are now 27 and 32 y.o.).
Both I and my wife started scuba diving at 16, because in that time (the seventies) that was the minimum age here in Italy. Later we both became instructors: scuba diving with air tanks and pure-oxygen rebreathers, swimming instructors, finned swimming instructors, and certified for salvaging both in the pool and in the sea.
After 5 years of experience in no-profit scuba training, we stepped-up to the "pro" world, and spent other 5 years as professional instructors n a top-level touristic operation organization, Club Vacanze, operating resorts in Italy, Greece, Kenya, Tanzania and Maldives. We worked mostly in Italy and Maldives.
In 1990 our first son was born, so we retired from pro activity. We started bringing him to the swimming pool when he was 6 months old. Around one year he was already swimming autonomously with small inflatable arm-sleeves and we started introducing him to fins, masks and snorkel. He did take another 6 months for mastering them properly. At 2 years he got his first small tank of air in the pool, and we started teaching him how to exhale air when ascending (from less than one meter depth, of course), how to remove the reg and place it in mouth again without drinking water, etc...
Around 3 years old he was already capable to perform all the basic skills in the pool: removing mask and evacuating, removing tank and getting it back on his back, buddy breathing with one of us alternating a single reg, performing simulated CESA with continuous exhalation, using different kicking styles, passing under submerged obstacles with touching them or the bottom of the pool, etc.
In the meanwhile during summer holidays in Sardinia, we introduced him to snorkelling and very light free diving, learning how to equalize ears and mask, how to evacuate the snorkel and the mask, how to swim with proper kicking on the surface and underwater, retrieving small objects from the sea bottom, etc.
Of course there was a pool also in the resort, but this was a deep pool, approximately 4 meters deep. So we started teaching him how to use the scuba tank in this pool, again mastering equalisation and exhalation during ascent.
At 5 years old we evaluated him to be ready for his first real dive in the sea, starting from the shore. We started in a very favourable site, a long slightly-sloping concrete slab where boats are pushed ashore, with a max depth around 3m over sand at the end of the concrete slope. This worked very well, so in the following two weeks he was capable to swim for some hundredth meter in open water, reaching a depth of 5 meters, where there were rocks and plants (posidonia) and a number of small fishes, an octopus, etc.
In the meanwhile we had our second son, who followed the same steps, a bit more accelerated, as he wanted to emulate his elder brother.
At around 8 years each of them had already something as 40dives in the sea, and were now capable to pass under short arches, and to stay perfectly neutral in water despite not using a BCD, only controlling buoyancy with their lungs.
They were certified OW by PADI at 12 y.o. A short time after the youngest one was certified, we went to our first overseas diving trip: Australia, the great barrier in Cairns. Here both were certified AOW during a LOB trip.
Back to Italy, a couple of years later (when they were 14 and 18) we finally did inscribe them to a serious diving school (the same where I and my wife did teach when young, before becoming pro), and they finally got the CMAS certification (we never considered the PADI ones to be "real" certifications as autonomous divers, capable of planning and conducting a deco dive, which is what we consider "normal" for a diver here in the Mediterranean).
Conclusion: in my experience, a child should begin as soon as possible. But the instruction must very VEEERY slow. It takes hundredths, perhaps thousands of hours in the pool and in the sea for building "aquaticity" and full self-control.
Everything must be taught as a game, making the child happy and curious. Going very slowly and progressively, we never faced anything dangerous or scaring. The child should never to be forced to do anything for which he or she is not ready.
I think this is really possible only when both parents are instructors: as a parent I could not have thrusted anyone else of me or my wife for being responsible of the life of our sons.
Only at 14 y.o,. we handed the youngest one to another instructor.
Some people here thrust instructors for teaching their son at 10 or 12 y.o. - too early for me.
Th problem here is not the kid to be ready, the problem is the parents to be ready to "leave him/her go" under the supervision of another adult, practicing a sport with inherent risks.
Here a photo of my elder son while diving inside a small cavern in Capo Caccia, Sardinia, summer 1998, when he was almost 8 y.o. (he is born in September 1990):