I think that age as a number is not nearly as important as health. You can be in poor health and be pretty miserable at 40 or 50. If you're in good health, you can enjoy things into your 80s.
I think the only time I doubted myself was my senior year in high school. I was not offered a Division I scholarship. I remember a scout from Ohio State coming in and looking at my film. He was all excited to meet me. Then he met me and I was 5'10" and he said that I was not a Division I quarterback