There are statistics on these sort of things. You might be interested in the book Real Age. The authors have a website too. For instance there are stats on how much exercise adds years into your life and how much shortens it. It’s all averages, and some of the information could be argued to be correlations and not proven to be causation, but it’s still interesting. Too much exercise by the way actually shortens life. Again, it’s averages, and can’t be a predictor for a particular individual.
People who sit all day probably do tend to have more health problems and die younger. It wouldn’t surprise me at all. There was a very old study about the health of mail carriers and that the ones who had routes with lots of walking overall were healthier than their counterparts in mail trucks.
I would think people in wheelchairs probably do have shorter lives on average. I know there is some stat on leg amputees also. Double amputation above the knee predicts a much shorter life than below the knee. I think it has something to do with circulation and the heart. I don’t really know all the medical reasons.
Mercola is very controversial. I agree with him on many things, but I also think anything he says should not be taken as the gospel. Do more research on your own. He is very extreme and often is contrary to the medical establishment. I often am disgusted with the medical establishment, but I try to look at research and also specifically what happens with my own body.
I try to make sure I don’t sit more than two hours without getting up. When I travel I take an aspirin and still try to follow the two hour rule.