Based on people I interact with most at Steemit, I'd say a very large number of Steemians suffer some kind of chronic illness - and that may be what allows them to find time at a computer. They (we) are not out climbing mountains, running marathons, plowing fields - we adopt hobbies that do not require that much mobility. Writers for centuries kept dying of tuberculosis, and now Steemians seem to be a community with a disproportionately large number of people with chronic health conditions. It's a no-brainer, if you ask me, a foregone conclusion, no need to even collect statistics - but math and accuracy were never my strong point. Sweeping generalizations, yes.
Now, a generation of gamers and computer geeks (no connection to Steemit required for this theory) may have become physically less fit due to their interest in spending hours and days on end at the computer, instead of getting outside to run like the wind and swim and get fit.