To dredge up a slightly old thread, I'll simply say that I agree with the majority of the comments above regarding shortsightedness in looking at age numbers only vs. actual participation. However, as a "young" (31 years old) guy looking at getting into the sport, I thought I could offer the younger demographic "outsider" perspective.
I work at a pretty bustling bike shop in Dallas, Bicycles Plus, and we see ALL kinds of business. Family bikes, hybrids, racers, wannabe racers, MTB guys, charity ride folks, $16K Colnagos and $700 Trek alloy road bikes, etc. Everyone on staff (except for one guy nobody likes) is an avid rider. Most of them do the local crits, as well as the winter cross series and one heavy offroad endurance guy and another track racer. In my roughly 1.5 years there, I have seen exactly one full-on proper touring bike. About half of the staff have steel bikes, but I'm the only one whose main bike is a steel roadie. I recently purchased a cross bike, intended to be a do-it-all commuter/adventure type bike. When I put real metal fenders on it, everybody freaked out. Talking about dynamo lights invited some light ribbing.
What I'm getting at is that in the whole of the "mainstream" cycling scene, randonneuring is a thing that lots of people haven't even heard of. Or if they have, it's an old guy thing. I only started looking into it seriously because I am not into crit racing, but do like long rides. Not that I have time for them at the moment, but if I did, it would be my preferred organized activity vs. road racing. When I say that people say it is an "old guy thing," that doesn't mean they'll never do it, but it does mean they have little to no interest in it at the moment.
Another thing is the equipment and ettiquitte vs. roadie group rides.
On the equipment side: yes, you can do a rando on anything, but there are lots of bikes that are pretty ill-suited for the purpose. Such as, you know, the majority of bikes we sell. Even an entry level road bike is so oriented towards road racing that it makes for a poor distance bike. And the bikes that are engineered towards being distance bikes still don't bother with fender and rack provisions, something you tend to see as a minimum requirement for most people doing rando rides. And the majority of lights folks are using, especially as things trend towards USB rechargeable lights, are pretty much worthless if you need to get through a full night of riding. So anything past a 200K ride, and you are looking at some significant to serious equipment changes, which can add up fast. Notice I haven't even started talking about entry fees, time off work, and hotel rooms.
On the etiquette side: I was pretty mystified when reading up on rando stuff regarding lighting. For roadies, it's a front and a rear blinky, in low-light situations only. Maybe a nicer headlight if you want to do some night riding. But when I first started looking at ride rules online, I found out that blinkies were a no go and reflective vests/sashes plus some sort of reflective ankle bands were required. I don't even go that far when commuting. I'm not complaining, it's totally a "when in rome" type of thing. I'm just trying to illustrate the big differences in what most folks are doing and what rando folks are doing. So beyond an age/time/"money" gap, it's a really foreign concept to the majority of serious riders in the U.S.
But on the whole, I would again point to the previous comments of others and say that the overall trend in cycling is growing and as the current "young guns" tend to age and drift away from crit racing, I'm sure a portion of them will find themselves looking at randonneuring in the future.