Every child is different. No two are alike. This article tells us nothing about this little girl. I can only speak from my own experience. At 9 at summer camp my son did a
PADI Program, think it was Seals. Anyway, he comes home and for his 10th birthday, the following week, he wants to get certified. Now 40 years ago on my one an only scuba dive - I hated it. Fortunately my brother is a diver so I thought he could go with my son. My wife, on the other hand, had other ideas. She signed us both up for lessons. Thankfully the eqipment has improved a bit in 40 years so it wasnt as bad as I expected. My son, who was an excellent swimmer was at home in the water - too much so for my liking. He always did his buoyancy checks upside down because "your floating dad, doesn't matter which direction you face".
I will admit that after he got his OW and even his AOW, while I was his dive buddy - he really wasnt mine. But he decided he wanted to work towards his Jr Master Diver and he wanted to get it as close to his 12th birthday as possible. It was taking the Rescue Cert that I first saw a dawning in my son that what we were doing, while fun, was truly dangerous. After that class there was a remarked change in his attitude. I could see he was starting to dive as my buddy, not as my son.
At thanksgiving, we were planning dives to work on some specific trim issues and to get those dives needed for his JMD cert. A cold front came in and the water temperature dropped to 57 degrees. We had 5 mil wet suits with boots, gloves and hoods but it was cold. We had been diving at 63 but never below 60. Anyway we thought we would try it. It was cold but we managed our first dive without blue lips or chattering teeth so we thought we would do a second. About 10
minutes into the dive, my son signaled to surface. When we got there he told me he was too cold and was calling the dive. Now we always said anyone can call a dive - but had never been diving when one was called! But he called it. It was at that point I realized he would be OK. If he could call a dive he really really wanted to complete, he truly understood what it was to be a safe diver. I (and his instructor) was very proud of him. Subsequent to that dive we have had a couple of miscues, got separated once on a blue water dive and after counting my 60 seconds I surfaced - he was there waiting for me. He just counted quicker.
So yes, by 12 I felt he was a safe diver who understood the risks and the planning needed to make safe dives. But if we hadnt been diving he wouldn't have been ready. It wasnt his age that made him ready, it was his experience and his understanding of the risks. You am only truly get that, at any age, by experience. So it's true some young people are ready and some aren't. But to say everyone is not ready by a certain age may be an overstatement.