Forever 75 You celebrate your 75th birthday in good health. As far as you can tell, you are a spry, relatively normal 75-year-old. And this condition does not seem to change over time: When you celebrate your 80th birthday, you look and feel exactly as you did on your 75th. When you hit 90, you still look and feel 75. On your 100th birthday, you realize that most of your friends are now dead or dying -- but physically, you are the same independent person you were 25 years ago. You hit age 110 with the same results. Every year, you are celebrating a new birthday without physically evolving beyond the age of 75. Doctors have no explanation for this inexplicable stasis. At what age do the thoughts of immortality pop into your head? How old would you have to be before you would start to seriously believe that you are immortal?