A while ago I read an article on Cracked.com about why you wouldn't want to be immortal. Among many other reasons was that time would speed up for you after so many years and it would make every second seem fleeting. Relative to a year old, each day is a long time because it's a large portion of their life, but to someone who has lived 100 years, it's a very small fraction.
How though, would you calculate what true percentage a certain amount of time is of someone's life? For example, what percent of a two-day-old's life is day one? Day two?
I have no clue what to tag this, but I'm hoping someone who knows what this kind of mathematics is called can retag it.