Doesn't that sound extremely weird? Like, how does puberty, and 9 months to have a baby, fit in with a 12 year total life expectancy?
Maybe I need more coffee or something...
When you look at pre-modern societies (essentially until around 1900 or so), there are four large causes of low life expectancy that don't really exist anymore. Infant mortality was extremely high (a quarter or so of live births didn't make it more than a few days). Then child mortality itself was high (you had perhaps a 50-50 chance of making it to puberty). For young women, there's an extra hit to life expectancy due to risky childbirth (you have about a 10% lifetime chance of dying in childbirth). For young men, they get extra risk due to endemic warfare--the baseline casualty rate of premodern warfare is basically WW2-levels of total war. Of all these factors, the infant mortality is the heaviest drag on life expectancy, as dying at age 0 really pulls down the average lifespan.
If there are 10 births, where 9 of the children die in infancy but 1 lives to be 100 years old, their average life expectancy would be about 10.