This is a contentious issue.
I have to admit that from the medical point of view I would not recommend scuba as a sport for children : the potential medical consequences are far too great.
The South Pacific Underwater Medical Society does not recommend children below the age of 14 present for training, but I would suggest that around 16 should be the earliest age.
There are numerous reasons for this :
Maturity - it's hard enough to tella crusty old diver like myself not to dash off after some fast moving critter and pursue a hazardous profile of sharp ascents and descents let alone a youngster,
Stress- while our kids are the smartest in the world, a mistake or misstep is unforgiving in this environment and can easily occur when a child is overwhelmed,
Medical- there have been no organised studies in kids to evaluate the risks of diving. This is not an invitation to therefore pursue it, in a vulnerable population, but to be thoughtful and circumspect
Further, the potential for microbubble injury of growth plates can impact growth, undiagnosed PFO's can contribute to neurological injury, underdeveloped eustachian tubes can contribute to middle/inner ear injury, and it is unclear what repeated microbubble loads do the brain. Is the risk benefit worth it?
While skeptics would say that there are no studies so to heck with it ; let's get them going, the truth is, the absence of evidence does not mean the evidence of absence in terms of minor subclinical injury with long term consequences that emerge in later life and are attributed to other conditions.
I believe that a child should be mature both physically and mentally prior to pursuing our sport. I realize that many will say, "my child is only 12 and the best dive buddy I have ever had. He/she is far better than divers 3 times his/her age". And I am sure that such divers are out there. I just cannot help but wonder about the prudence of putting kids at such risk when safety and resposnisibility should be our prime focus. Additionally, I can only feel a sense of cynicism at the commercial agencies that have found another potential market to tap into. This is not a criticism of those parents who wish to pursue this, but a moment of pause to fully consider the ramifications of what we do as adult divers. I can think of nothing greater than diving as a family....when the kids are ready.
Safe diving.