My oldest daughter learned to dive at 12; my youngest is preparing to take her course this spring and she is almost 12.
They are both very mature girls.
However, when deciding whether or not to allow our daughters to learn to dive, I agreed with limitations. When we go on a dive vacation, they will do no more than two dives a day; they will not dive every day; and over time we gradually expand their depth limit to 40 feet maximum depth (even though oldest daughter has AOW certification); and when they do dive, we hire a dive professional to be their buddy. I believe not having Mom and Dad there to make all the decisions for them will make them a better diver in the long run. I also believe we ALL enjoy the diving more if I'm not hovering over them during the dive.
I don't know if those things above are the "right" things to do; I don't know if they are "enough"...what I do know is that my daughters both BEGGED to learn to dive. We let them do things every day that put them in harms way (example, my daughter is a pitcher for her softball team...any idea how frequently pitchers take a ball to the head??). And I"m raising them to live life to the fullest, never letting FEAR stop them from pursuing their dreams. We give them the tools/training/experiences to help them become the safest divers they can....and then we pray. A-LOT of prayers. A-LOT!!