Brand new diver here, so perhaps can provide a different perspective to the topic than many of you who have been diving for quite some time.
My wife and I both just got our open water cert and have done just a handful of dives. We are both in the "empty nester" stage of life - more time, more resources, more interest in doing things we want to do and not just wrapped around our kids activities.
However, we wouldn't have taken it up if we didn't have friends who were divers. Yes, we had interest in exploring the world underwater, but the tipping point was more social - going with friends on trips and sharing experiences.
There is a broad range of entry costs - you can get started with some time and minimal equipment and fees. But in general, its an expensive hobby (compared to many others). Even if you rent gear, the tank and dive fees can add up. That certainly may be a barrier to many. That may say that it's better suited to those a little older who have the means.
On the flip side, on our first two boat dives after our certification, there was a very diverse (in terms of age) group of people on the boat.
I think Tursiops was onto something in his post about "flow" and having the skill level to make it fun:
That is, not enough skills for the challenge produces anxiety…too much skill for the challenge produces apathy….but a match is a necessary condition for Flow/Zone.
Diving has a certain amount of challenge attached to it, and deep or high-currents or low-viz or beyond NDL or photography can all raise the challenge beyond your skill set…and thus produce anxiety. Being anxious is NOT fun. Being apathetic and bored (too much skill for the dive) is NOT fun.
As a new diver, it's not yet "fun". We are still thinking through all the techniques and gear and technical aspects of having a safe dive. It's not automatic. Many have told us that kicks in at 25 or 50 dives. But that takes some pretty good staying power for new divers to get over that initial learning period.
The analogy is driving a motorcycle. I took that up about 5 years ago. The first 2-3,000 miles were all thinking; about bike mechanics, positioning, environment, etc. It took a lot of concentration. But now, confident and comfortable in my skills, I truly enjoy the ride. I suspect diving is similar. I wonder how high the dropout/quite rate is before getting to "flow"?