First, a little preliminary discussion on life expectancy. "Life expectancy" is defined as the average age of death, or the average years that individuals will live for. For "life expectancy at birth", or LEB, or LE0, these definitions are identical. We can also use the life expectancy at a later age. For example, the life expectancy at 20 (LE20) is the average age of death for those who have reached the age of 20, or the number of additional years that individuals will live for, on average. These two numbers will differ by 20. When reading papers/books/websites discussing life expectancy, it's important to know whether the numbers they give are the average age of death or the average additional years (except for LEB/LE0 for which they are the same). Below, I give the average age of death.

A convenient life expectancy that excludes infant and childhood mortality is LE15. The average LE15 for a selection of modern hunter-gatherers is about 53 (Gurven & Kaplan, 2007; Kaplan et al., 2000), varying from a low of about 44 to a high of about 58 for the different groups studied (Gurven & Kaplan, 2007). There are still significant numbers of deaths in adulthood due to disease, accidents, and violence (homicide and war), and the LE45 is significantly higher: an average of 66, varying from 59 to 69.

LE0 varies a lot between different groups, from about 21 to 37. Infant/childhood mortality varies a lot between different groups, and malnutrition and infanticide also affect it.

Overall, in the groups studied, about 70% of deaths were due to disease (much of this in infancy and childhood), 8% due to accidents, and 13% due to violence. Deaths due to violence (homicide and war) vary greatly between the different groups.

Can these patterns of mortality see in modern groups of hunter-gatherers be extrapolated back in time? Probably. Hunter-gatherers with settled agriculturist neighbours appear to suffer higher rates of infant/childhood disease, with infectious diseases that are endemic in the settled population but would become extinct in small hunter-gatherer groups being passed from the settled people to the hunter-gatherers, but the impact of such neighbours on the LE15 and LE45 is much smaller.

Data from 18th century Sweden also shows LE15 and LE45 very similar to the modern hunter-gatherer values: an LE15 of 55, and LE45 of 65 (Gurven & Kaplan, 2007).

References:

Gurven, M., & Kaplan, H. (2007), "Longevity among Hunter-Gatherers: A Cross-Cultural Examination", Population and Development Review 33(2), 321-365. https://www.jstor.org/stable/25434609

Hillard Kaplan, Kim Hill, Jane Lancaster & A. Magdalena Hurtado (2000), "A Theory of Human Life History Evolution: Diet, Intelligence and Longevity", Evolutionary Anthropology 9 (4), 156–185. https://doi.org/10.1002/1520-6505(2000)9:4%3C156::AID-EVAN5%3E3.0.CO;2-7