My regular dive buddies run the spectrum from late 20's to early 60's. I'd like to have a couple in their teens, but I don't think I can get the students from this spring back in the water until school lets out
Diving, as it is promoted today, is for the majority of people a travel activity. Travel is expensive and requires time. Young people don't have much money and often don't have much vacation, either.
If you are lucky enough to live where I do, diving is a local, relatively inexpensive and year-round activity, which is much easier for young people to get into. And then they marry and start families, and the TIME becomes the limiting factor, and at best, they are vacation divers for a number of years. (That was my husband's trajectory, too.)
In the last three months, Peter has certified at least six people under the age of 20. There IS interest in that age group!
My college Scuba classes are full every semester. That is 5 OW classes per semester, plus Scuba 2 (a very comprehensive AOW Course.) Fall 2010 plus Spring 2011 totaled about 250 students between the ages of 18-23.
At the recent PADI update, we were presented with figures which showed the largest number of new certifications world-wide between the ages of 19-29.
IMO the most active divers do tend to be middle aged due to more leisure time, more disposable income, and because diving is fairly cerebral and less physical, which enables the athletically declining to partipate on an equal playing field.