I found, on the Wikipedia article for life expectancy, that the life expectancy of those in the Neolithic (and Paleolithic) eras was around 33 years, which was one of the highest numbers up until 1900. I am having trouble believing this, because of the counter-intuitive nature of the information, and because it's wikipedia. Is it really true that the majority of human progress until after 1900 was detrimental to human life expectancy? And why was that the case?
It's a difficult question because only a small percentage of human bodies survive, and many of the remains survive only in very fragmentary condition. It can therefore be difficult to accurately determine age (although, it's fairly obvious when you're dealing with an infant or a young child). Lifespan estimates for prehistoric periods are therefore largely hypothesised based on other factors such as ethnographic research of more contemporary cultures.
It is also generally assumed (with good reason) that child mortality was much higher than in our society, and more women than now died young in childbirth due to the lack of gyneaological medicine. These factors have a big impact on the average life expentancy estimate.