Infant mortality in hunter-gather societies makes the average life expectancy of 30 years seem low.No, not really. Yours survivial totally depended if you was able to find food. If not you was screwed. And there was extrmely high mortality level and basically no one couldn't live old age. Thirty years old was considered as old. Yours only security is only just yours own tribe, nothing else. And if you got sick or injured you probably just would die quickly and painfully. Yes, there would be lesser pollution and violence but it hardly is worth of dying young from reasons where you probably wouldn't die on modern society.
A lot died before the age of 5 but if you lived past that you had a good chance to live into old age.
They tended to be spread out over larger areas so had less contact with each other so less disease transmission.
It takes more work to grow crops than to hunt and gather until mechanised farming comes long.
People start farming when become harder to find food and it becomes worth the extra effort to grow the crops.
Hunter gathers lived just as long as modern humans. the difference is now fewer people die when they are young.
Farming supports much higher population densities and makes the devolvement of cities possible.
The downside to early cities is the more crowded conditions leads to more infectious diseases spreading in the relatively crowded conditions.