The average life-span was actually quite low, not that low, but due to child mortality the average life expectancy has been suggested to be about 18 years. But that simply means that a lot of kids do not make it to adulthood. Once people made it to adulthood the life expectancy was around 30 and 40 years.

See: "A Demographer's View of Prehistoric Demography [and Comments and Replies]" William Petersen, Robert J. Braidwood, Henry F. Dobyns, Wolfram Eberhard, Robert E. Kennedy, Jr., Gottfried Kurth, Christopher Meiklejohn, Yoshio Onuki, Kenneth M. Weiss and Paul F. Wilkinson Current Anthropology Vol. 16, No. 2 (Jun., 1975) , pp. 227-245