I started when I was 29. It helped that I had just gotten out of the Army and was flush with deployment pay, (I had also just gotten back from Iraq.)
I think the monetary bar to entry inhibits younger divers, but I think there's a perception issue, yet I can't seem to lock onto what that is.
TraceMalin has posited here in the past that it's a lack of "macho" and "demanding" training, and that young people are drawn to such activities. As a young person who craves adventure, I do agree, (though the position is not popular).
I also vintage-equipment dive, and on the VDH forum, they similarly suggested that a lack of a modern "macho-image" Mike Nelson or explorer Jacques Cousteau, and a "foo-fooing" of the sport via modern equipment, (all dangly, bright-colored poodle-jackets versus "manly" tank harnesses, huge knives, black everything, etc.), along with modern training taking some of the "adventure" out of the sport, (IE, training is too "namby-pamby" and "lowest-common denominator-oriented".) I also agree, though the position is not popular.
I think there is something in both (related) arguments, yet I don't think they hit the nail 100% on the head. I do know that when I go diving locally and encounter divers and non-divers alike, I get WAAY more response out of my vintage double-hose rig than I do my modern kit. More questions, more interest, more "wow, that looks cool!"
Now, I'm not drawing any conclusions, but I think part of the problem is that the money needed, (or perceived to be needed) to enter the sport is high, and so young people only see older people doing it, (since they can afford it). This leads to the perception that "SCUBA is for old people".
I teach diving on a college campus, so 99% of our audience is in the 18-25 age bracket, and money, or lack thereof, is a HUGE issue. They may love diving, but they can't afford it, (or think they can't. I had a student complain about the cost of a $40 mask, saying it was too expensive, so he didn't buy it. Then that very night, I saw him at the pub drop well over $100 on drinks for him and his buddies. Priorities, clearly.)
But I think more importantly is that diving is competing with other activities and it can't market itself as attractively. Scuba is competing with horseback riding, dirtbikes, rock climbing, skydiving, and other "adventure sports". The problem is that diving is perceived as a "travel sport", that the only good diving is on vacation and you have to travel to do or see anything decent. Too little emphasis is put on local diving.
So people figure, "I can get a harness for a few hundred bucks and go climb rocks all over the place for free."
Or, "A few hundred bucks on a used dirtbike and I can hit the trails in the local BLM area for free."
Or, "I can spend hundreds of dollars to learn to dive, spend thousands on equipment, then I NEED to spend thousands on vacations to actually DO this activity."
Wrong, but that seems to be a common perception. So which one wins?
I don't know what the answer is, but I do know that diving needs a "cultural shift" away from once-a-year-vacation-diving-is-the-only-good-diving, or the mean age of the participants will continue to climb
($$$$$$$) and the sport will continue to see the high-attrition of the 95%-quit-within-five-years drop out rate.
I also agree the image of scuba needs help, but I'm not sure what that is. We'll never see a return to pushups on the pool deck in full kit, double-hoses, duck feet and BC-less harnesses, at least outside of niche corners of the dive world, so I'm not suggesting that.
Again, I don't know where we're going, but I do know that I'm gonna keep diving, in as many places as I can, in as many ways as I can. As long as I can dive, and share my passion with others, I'm happy.
Sorry for the long post, but this is something I wonder about a lot, but am constantly flummoxed as to the way forward. I know my local "dive peers" are all around my age group, but we're in the tiny minority. Most are older than we are.