The length of a human life has usually rested around 70 years, which is actually seen in nomadic people today. Their life expectancy can be appallingly low due to their environment, it doesn't mean everyone's dropped dead by the time they're 40, it means lots were lost at a ridiculously young age.
During the industrial revolution 75% of children in London died before 5 years old. This meant that life expectancy was only around 30 years, despite the fact that poor houses were predominantly old people. The UK instituted the Old Age Pension Act in 1909 to deal with this problem and the claiming age was 70 years, which means people were expected to work until this age. If people were really only living until 30, why would people have invented a pension for 70 year olds?
IIRC the breakdown for the Paleolithic was, at birth life expectancy was ~33 years. If you lived to 15, your life expectancy was, on average, actually between 39-55 and I wouldn't doubt that if you lived to 55 you had a solid chance of making it to 70.
During the Bronze Age life expectancy at birth was 18, this was due partly to wars, population densities and essentially the plagues caused by such (this is the essential reason why Europeans had diseases that wiped out 70-80% of Native Americans). At about 1000 BC we hit our average of 30 years again, although for Greece and Rome it actually ended up being 20-30 years because again of wars and some diseases (although by this time we were more disease resistant).
Another example of this is the USA. People in the US have a lower life expectancy than many other countries, which aside from my problem with privatized health care (which is primarily a quality of life, not a quantity of life issue), is mainly due to the US having an appalling infant mortality rate... however if you actually survive infancy you're good to go.
IMO stating life expectancies is very misleading, due to the fact that it creates the impression of a Logan's Run environment where you're dead at 21. Only in modern society has life expectancy surpassed 40 years, but Genghis Khan died at age 72 (IIRC) due to an infection after his horse bucked returning from war. Apparently this was no mean feat in the Mongol armies as many of his generals were as old, if not older, than him as they'd been with him since the beginning.