The November Discover magazine pg. 16 has an article discussing the above. The author found a relationship between the three. He equates the ease of sitting and standing unaided on a smooth floor with longevity. The more assists with hand or knee, forearm or thigh, the shorter the remaining life span.
I have been going in and out of seiza for 21 years because of the martial arts I practice.
BTW, I just turned 83.