I didn't read all the replies, but I did scan several. I'd venture three things. First, there's probably a larger nude cohort here given the site and the types of people it draws (well-endowed men and/or those who are more than a little interested in penises and larger ones). Secondly, I'd say that those comfortable nude probably are aged 30 or above. The younger guys seem to be less comfortable for a various reasons (which you can find discussed at length on various forums on here). Third, this nude in steam and sauna must be on the wane in part due to rules. At the gym I go to they have posted regulations that one must have shorts or swimsuit on in steam room or sauna.
For me this takes away the point of the steam which is no clothes and letting the sweat just roll with the steam. Admittedly most of the guys in their 40's (my age in other words) or older don't necessarily follow the letter of the rule. They seem to take a "if I have to cover I will" approach.
The rule was made to eliminate "inappropriate" behavior, which was jacking off in the steam room or having sex in there (I assume). Better to deal with the less discreet and let the rest of us have the small pleasure of a naked steam in the company of others who enjoy it, too. But, whatever.
I hate to see this disappear, but I'm afraid it most likely is on the way out. Too bad!
In answer to the OP questions, though, I'm 47 in Oklahoma and I'm nude in both if I can get away with it.