Pretty sure the probability of making it to adulthood has never been below 50% excluding war, plague, or famine (which were common, so hard to normalize)
But if 50% reaches an average age of, say 2, assuming it’s right skewed for those who died before 10; then an average life expectancy of 50 for the remainder means the average life expectancy overall just 26. That squares with the numbers stated I suppose.
For example, for Ancient Rome it says “ while the ~50% reaching age 10 could expect another 40 years of life”.