They are in Rome on 640k. Basically, Granaries, Aqueducts, Medicine, clean power plants, and Recycling Centers increase life expectancy. The late techs that introduce population-based pollution can reduce it.
life expectancy =
1500 / ( 20 + ( ( 50 * Pop / ( Pop + GranaryPop + AqueductPop ) ) * 0.5 If Medicine ) ) - ( Pollution Tons * ( 10 / Pop ) )
Pop = sum of all cities' size
GranaryPop = size from all cities with granaries
AqueductPop = size from all cities with aqueducts
If Medicine is discovered, a modifier applies in the expression's denominator, reducing it.
Pollution Tons =
10 * ( Shields / Buildings - 20 + ( Tech * CitySize / 4 ) )
Each city runs this and the results are tallied for the civ.
Buildings = 1, 2, 3 respectively for Default, having hydro/nuke plant, having a recycling ctr.
Tech = 0, 1, 2, 3, 4 respectively for Default, IND, AUTO, MASS, PLASTIC i.e., no pollution from population until Industrialization, and the problems increase with certain benchmarks