The hubby and I are members of a gym near our house, and my estimate is that membership at the gym is about 40% over 60 years old, 30% are 45-60 years and and the other 30% spread out from are below 45. I could be very wrong of course ;)
There is a cozy indoor heated swimming pool and the average age of swimmers is about 70. Yes, that means wrinkly cute elderly folks. I think its great that they are exercising, socializing, going for these waterobics classes and happy in their bathing suits. I hope to be as snazzy when I'm old. The heated pool attracts them because its warming to the bones and has low impact on them joints and stuff. Pretty ideal exercise for that age group.
There are land exercise classes too, and I'd say most of them are about 40, with a handful of 20-somethings and a handful of 60-somethings. Conversations you overhear go like, "After my fourth child..." and "My grandchildren...". This is not to say the class is easy, in fact, it gets you pretty sweaty, and sometimes push-ups and sit-ups go up to a hundred. The instructor for this 100 repetitions class is probably close to 50 (seriously) and definitely a lean bean. She does Pilates too and while I start shaking in the first few seconds, she's asking the class if we want to go some more. The 40 something year old in the front row says, "More! More! Roar!!!" Obviously, I try to grunt as quietly as possible in the presence of these muscle-women, who are about 20 years older than I. So much for slowing down when you get older.
There is a pretty big weights room upstairs and various treadmills, stairmasters and other torture devices. The age up here is varied, but when someone says, "Excuse me young lady", he or she is usually well-qualified to do so.
So, I stick with my estimate.