Do castrated men live longer than non-castrated men?
Studies show that castrated men live longer. The earlier the castration took place, the greater the difference in lifespan appears to be.
Castrated men live longer than non-castrated men. That picture emerges from studies in both people and mammals kept in captivity. The survival advantage is greater when castration takes place before puberty, which suggests that the hormonal drive to reproduce comes at a cost to lifespan.
The effect does not appear to apply to non-surgical ways of suppressing testosterone; only actual castration shows a survival advantage in men. In women the picture is different: permanent surgical sterilisation is associated with a slight decrease in survival. Male and female reproductive hormones therefore do something very different when it comes to how long you live.
But the picture is not as simple as 'less testosterone is better'. Low testosterone levels in men are associated with obesity, metabolic syndrome, type 2 diabetes, and cardiovascular disease. Testosterone is therefore not purely harmful; it plays a nuanced role in health. Animal studies confirm that male sex hormones influence energy metabolism and body composition in multiple ways, and that the outcome depends strongly on the organ and the measure you look at.
In practical terms this means: castration probably extends lifespan in men, but it is an extreme intervention with wide-ranging health effects. The link with lifespan does not mean that low testosterone is generally healthy; for those who do not choose castration, the opposite is more likely to be true.
This answer rests on two large summaries of earlier research. The finding that castrated men live longer is consistent across multiple species and has also been seen in people, but these are associations, not controlled experiments in humans. The evidence for a survival benefit from castration before puberty is stronger than for castration in adulthood. The apparent contradiction between the survival benefit of castration and the health risks of low testosterone is real and acknowledged in the research; the researchers themselves point to the complex role of male sex hormones.