dvcochran
Grandmaster
Sedentary work could be a factor, but we live in a very blue-collar town and the large percentage of our adults work outside or in a factory environment. Some are 'stiff as a board', some are naturally more flexible. I cannot say there is a ton of predictability in flexibility. Even age is not a hard predictor, but it is very apparent that the older we get the less value it has in a sustained encounter (sparring & such). This is very much where muscle and strength come into play.I think with folks who sit for work, it's necessary for all styles, to varying degrees. I try to remember to leave time for at least a few cool-down stretches, but I don't always notice the passage of time, so it gets missed.
To your point on toughness, there may be more 'hardening' with some of the blue-collar people vs. white-collar folks but that is not a hard predictor either.
For the people who really want to get into it and learn and grow, background may not have a ton to do with it. It is what they want to do/be going forward.