h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Either I'm misunderstanding it, or there must be a mistake in Footnote 1:

"[life expectancy is] The average number of years you can expect to live. For statistical purposes, it refers to the total number of years everybody with the same age as you will live, divided by the total number of people of your age: that number of people in your age-group being limited to a country or region."

The bit that puzzles me is "the total number of years everybody with the same age as you will live". Given that nobody knows that, it must be a number based on life expectancy - so to work out what the life expectancy is, you need to know the life expectancy already, and then divide it by the number of people of that particular age.