It could also just be due to individual variation in the study subjects. Health seems so individual in people of a certain age that I think you'd see a lot of variation regardless of what treatment you proposed. Figure, not everyone will benefit from exercise, some people will still need walkers or whatever into their old age and no amount of tai chi would help them out. Like you said about the inflammatory tissue, it might just make an underlying condition worse. Regardless, I really want to take up tai chi when I'm older.