I say that a figure like that is an estimation of average lifespan based on modern day hunter gatherers, if based on anything at all, and the problem with average lifespan is that infant mortality drags it down considerably. In a hypothetical population of two people where one dies before its first birthday and the other dies at age 90, the average lifespan for that population is of course 45 years. If you survived infancy, chances are you'd live a pretty long time. And infant mortality is fairly high in most animals anyway.
People seem to think that malnutrition and starvation were the biggest killers. They killed us back then just as often as they killed any other animal, if not less (I'm speculating) since we are omnivores. They don't make the connection that they can eat a healthy hunter-gatherer diet while not actually living like a hunter-gatherer.
You lousy kids! Get off my savannah!