when I'm in some accident and become a quadriplegic (god forbid), that would be right about when I'd stop training.
Why stop ever? I see people over 60 at my gym, and plenty over 50. Some of them, those who've been doing it a while, look great. Unless you develop some medical condition where it's dangerous or impossible to train I wouldn't quit just when I get old. Training helps keep you feeling younger, so they say. If I'm still training fine by then, I'm not going to just stop on my 70th birthday and say, Well, I guess I'm too old now, time to quit the gym and just sit at home.
No way!