It's suspected that there is a biological trade-off between reproduction and longevity, the theory being that our mechanisms of repairing damaged genetic material are limited and thus relegated to the most evolutionarily advantageous repair work. Propagating our genes, it would follow, trumps living to see/attempting to control the lives of proceeding generations. The male sex hormone is implicated in this theory, and taking into account that fact that women tend to live significantly longer than men, may be responsible for limiting men's lifespan.
METHODOLOGY: Because sometimes you can't offer enough cash or college credit to put together a randomized control experiment, researchers did a (very) retrospective study of Korean eunuchs from the Chosun dynasty, which stretched from the late 14th to the early 20th century. The eunuchs, a class of nobles employed as guards at the royal palace, preserved their lineage through the adoption of castrated sons and kept detailed genealogical records, which the researchers cross-verified with other historical accounts.
RESULTS: Averaging a lifespan of 70 years, the eunuchs lived about 14 to 19 years longer than cohorts from a similar socio-economic background. The group of 81 eunuchs included three centenarians among their ranks -- making them 130 times more likely to celebrate their 100th birthday than, for example, men in the present-day U.S.