Remember that for Europaean church, it was important to christen all live born children, including those who died in infancy, and they often created records of that. But who cared in China?
On the other hand, Tang China ought to have detailed population data because they had well-field system.
Did the Tang censuses count only taxpaying adults, or all souls? In the well-field system, did unproductive mouths, such as children and elderly, get any land or tax breaks? At which age was an otherwise healthy Tang peasant officially old and liable to have his well-field land taken away?
The elderly was classified as 60 years and older. However, land was not reclaimed by the government until death. However, the amount of land was reduced as a person reached elderly status. As Victor Cenrui Xiong stated, "From the state's point of view, as the adult man became elderly male, his grant had to be reduced because he would become less productive and tax-exempt."1
The fact that there was a population classification that was at 60 years and older, it is evident that it was reasonable that people should live that long. One thing to remember when people talk about life expectancy, is that infant deaths are going to skew the numbers, so that it becomes much more difficult to determine just how long people lived. Of the 30 households studies in Xiong's paper, only four elderly men were the heads of households. That doesn't take into account elderly women, however.
1.Xiong, Victor Cenrui. 1999. "The Land Tenure-System of Tang China: A Study of the Equal-Field System and the Turfan Documents." T'oung Pao, Second Series, Vol. 85, Fasc. 4/5, pg. 372. Brill