In my opinion, 'average life expectancy' is redundant, because a life expectancy is based on an average lifespan (i.e., average age). As a human being, our maximum lifespan, according to scientists, is 120. From the context of your sentence, the average lifespan / age of Okinawan is 81.2. In other words, 81.2 is an expected age that an Okinawan can live; hence, their life