In my experience you see this quite a bit in BJJ as well. I wouldn't say it's nearly as diverse as climbing, but it's definitely not all young athletic men. My son's classes were about 4:1 boys vs girls, and the adult classes had fewer women. But I got to experience being choked out by a 70 year old man and a 16 year old wizard on the mats. There were all sorts of fitness ranges, but typically those improved rather quickly for the folks who stuck with it. We only had a couple guys who I'd put in the religiously works out strength category. Those guys are legit scary when you know they have the same skills as the skinny 70 year old who just put you through your paces and can also just physically twist you into a pretzel on top of it just through raw strength. All were super friendly and inviting to new folks though.
You know, people say this a lot about "typical" weight lifting focused gyms, but I've found nothing but the opposite. There are certainly some people who are in there that are judgy (the are everywhere) but the vast majority are either there to do their workouts and leave, or are very friendly. And I've done most of my training in gyms in Manhattan and Paris, two places not known for friendly people.
If you go to the same gym you are already in-grouped. It is the insecurity of the new person who would think they are out-grouped.
Meatheads love new lifters. No one showed up at their first gym 230lbs and ripped. Even the biggest guy in the gym can relate to being the intimidated new lifter.
The only difference I would think between a gym with weights and a climbing gym is that the people at the climbing gym look less intimidating.
Also - weight training is also a highly technical problem-oriented discipline. You can do it for years and still not have things working the right way. The techniques are evolving as well. So - lots of problem solving and discovery.
FYI - one of my favorite sites of all time and certainly the technical view on things: exrx.net
The people who talk shit about those people are generally insecure themselves and projecting their inner monologues on them - and the thing is, all the experienced people have been there and done that. Breaking through that is hard, but so rewarding.
I certainly have no basis in painting all climbers with this brush, but this was my experience, and I found it to be pretty bizarre. Completely turned me off to making climbing a hobby of my own.
I think that - unlike many gyms and other fitness communities - it's very welcoming to non-conformistist and people who don't look like typical gym junkies.
It's hard to have an ego when just about everyone gets outclimbed by 12yo girls, and the best climber in the world looks like someone who got fired from a big tech company because he looked too geeky: http://www.alpinist.com/doc/web16f/newswire-ondra-dawn-wall (and I love that even he failed to replicate Lynn Hill's "free climb The Nose in 24 hours" - apparently even Ondra needs to practice Changing Corners).
Also, bouldering in particular is such an intellectual pursuit. It's problem solving under physical pressure, and forces you to think hard about the geometry of your body and how weight works etc.