The experience with my sons (that started at 18 months using a small tank in the pool) is that children are very cautious and aware of dangers when aged 5 to 8. That for me is the best age for introducing them to dangerous sports. In fact both my sons had their first real scuba dives in the sea only after 5 y.o. - same for free diving, motocross, mountain climbing, alpine ski, etc.It's not odd. PADI/SSI OWD materials are already really dumbed down. There is a minimum you should know in order to dive safely. And that minimum pretty much is the OWD class.
Having taught a number of kids, I think 12 years is already very young... too young for some if not most kids. I would not do a class with kids under 12. The issue is not that they can't learn the motor skills (usually easier for them than many for many people over 50) or even the physics (even though some adults have issues with some of it), the issue is that young kids are not good at judging what's dangerous.
The danger when climbing, skiing, mountain biking, etc is very apparent to kids, in scuba diving, it's not.
Many of the +50 year old diver have too much fear and young kids too little. A kid can't really be a buddy, a kid needs to be watched by a very solid and attentive diver. IMHO, 20 - 30 feet should be the max depth for kids under 14 or 15...
After 9 y.o. many children become too adventorous and over-confident and tend to underestimate risks.
This goes worse and worse, the worst is around 14 y.o.: at that age many youngsters do not follow adult's instructions, they feel invincible and are in a real danger.
That is the reason for which I suggest parents to introduce their sons to dangerous activities when they are very young (5 to 8).
If they do not become proficient in these activities before being 9, then better to suspend the activities until major age (18).
Starting scuba diving at 10÷12 y.o. is a bad age, in my opinion.