Well, according to the cited articles on that Wiki page, with respect to the Paleolithic era, "Based on the data from modern hunter-gatherer populations, it is estimated that at 15, life expectancy was an additional 39 years (total 54), with a 60% probability of reaching 15."

Regarding the Neolithic era, "Based on Early Neolithic data, total life expectancy at 15 would be 28–33 years."

Interestingly, the average life expectancy was lower in the later, sedimentary and socially hierarchical era.

In any case, you're right that 33 isn't the average in the sense of the mode (the most common age in which death occurred).

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