I would like to drift this thread slightly in another direction ...
My Wife, 10yr old son, and I were all recently PADI OW certified. My greatest fears were that my son might not be able to master the tables & calculations required for the academic portion and that my wife might not be able to perform the various underwater skills required such as: mask flooding, and mask and regulator removal and replacement without panic and surfacing.
My son completed the academic portion with an 88% with no help or coaching from us. My wife then completed the water tests, at various depths, each time without failure. There were several other classes (from other dive shops) at the popular local dive site, and some instructors were struggling with getting their students to merely accomplish the mask flooding ONCE, much less on consistently on demand, so that they could pass them. I do not think that accomplishing the task once, under no stress, qualifies as mastering a skill. If you cannot do it several times, on demand, consecutively, without failure, you have not mastered the skill.
I would feel FAR more comfortable diving with my 10yr old son than with ANY of those folks that exhibit fear underwater. My son has been snorkeling since he was 6 and snuba diving since age 8 and has NO fear of the water getting in his eyes or nose. He is totally at ease when diving and would be an asset to a buddy diver in trouble. While in the final dive he saw another diver who was having trouble and re-attached their inflation hose to their BC which had dislodged during the dive (or never properly attached).
Lets not get too carried away with the generalities and stereotypes ... there are good and bad divers of all ages. Dont dismiss (or accept) divers strictly on age alone.