Pensions and life expectancy

The argument for raising teachers' pension contributions is simply that people are living longer. How do we know this?

Presumably we can look at the ages that people die at, which will tell us that those who were born roughly 60-100 years ago are indeed living longer. We could look at past data and see that the average age at death is rising at x years per decade, then use this trend to predict that by 2035 the average person will live to 103 (say).

This sort of prediction does remind me a bit of my own forecast in April this year, when after a careful analysis of the last fortnight's weather data I announced:

"Goodness, the weather's getting warmer and warmer- by Christmas it will be absolutely roasting!"