What do you mean with being terminally ill? Statistically speaking your body will give up at 80 years of age. This is without any special condition like cancer at a young age. You can calculate how long you would have left.

When planning your life you better take into account that you only have so many years left and even fewer productive years. Once you are over 25 you can slowly feel how things decay if you reach 40, 50, 60, 70 it goes downhill faster and faster. There may be dozens of years where nothing bad happens and then there are some weeks where your biological age increases by 10 years and it never really gets back to the level where you have been for so long. No matter what you do, no matter how healthy you are, how much care you take of yourself, one day it will hit you, too.