Interesting thread in that there is very good longevity on my mother's bloodline, but two great aunts in that bloodline are in nursing homes (in their early 90s). They are living in that there hearts beat, but not "living" in my sense of the word.
While I realize that as I age there will be certain things that I will not be able to do in my 90s that I can now do in my 50s, just like there are things that I can't do now that I could do in my 20s. But when it gets to the point that I can't take care of myself and can't even wipe my own a$$, then I think it will be time to go. Easy to say now though.
That said, I visited a neighbor today who is in his 90s, you can converse with and is still living on his own (though his wife is in assisted living because he is no longer able to care for her).