I actually used to have that limit set to 60, but then when I thought about it, I probably would still have some physical strength at 60 and still wouldn't want to die. But at 80 I can't imagine myself being other than a total wreck.
My grandfather is 89. He can't walk very far, he gets tired very easily, and he has arrhythmia - yet he is still very strong, and very active. Most days, he gets up at 5, works in the garden, or the woodworking shop, or on property maintenance, finishes up by noon, and watches television until evening. This has been more or less his routine for the past 13 years.
If you remain active through your life, I think you're much more likely to stay active as you're older. I intend to retire by 40, but I don't intend to be idle.