When do we get old

I'm a young 72, still working at the job I've had for over 30 years because I love doing it. Just bought an inversion table to help my back pain. I'm going through my Barracuda fixing small things I let go on the build. I just started body and paint on my friends gen 1 radical built bronco. But the guy looking back in the mirror is an old guy. Where did he come from?