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It's all relative, and despite the Constitution stating that all men are created equal, it only applies in the eyes of God. Everyone ages differently physically. While some retain good mobility well into their 90's with examples of a few centenarians even, others lose theirs much earlier.

For some their eyesight is even the reason for less confidence in navigating such things as stairs.

I seriously doubt that Hillary was ever practicing yoga or any other form of physical activity. As they say, a body in motion tends to stay in motion. So lifestyles also play a part in the mobility factor of individual humans.

Many of the people living into their 80's & 90's have done so despite being an invalid for many years. Such as my grandmother who suffered from MS and required around the clock care from an assistant for almost 20 years. A good portion of that time she was unable to even communicate. She was reduced to nodding her head to acknowledge her understanding of what you were saying to her, or her trying to communicate what she wanted or needed.

I just picked the 60's as that seems to be the age that I assume most start noticing their capabilities beginning to diminish. It was not meant to be taken as a blanket statement that applies to every human being, just as the age that meets an artificial mean since it varies based upon a lot of variables. I may even be wrong in picking that as the mean, but it was arbitrary since I do not know what the mean is exactly, and it was the age I started noticing it personally.