Castration had a huge effect on the lifespans of Korean men, according to an analysis of hundreds of years of eunuch "family" records.
They lived up to 19 years longer than uncastrated men from the same social class and even outlived members of the royal family.
The researchers believe the findings show male hormones shorten life expectancy.
The study is published in the journal Current Biology.
Castration before puberty prevents the shift from boy to man. One of the scientists involved in the study, Dr Cheol-Koo Lee from Korea University, said: "The records said that eunuchs had some women-like appearances such as no moustache hair, large breasts, big hips and thin high-pitched voice."
Eunuchs had important roles in many cultures from protecting harems to castrati superstar singing sensations. The imperial court of the Korean Chosun dynasty used eunuchs to guard the gates and manage food. They were the only men outside the royal family allowed to spend the night in the palace.
They could not have children of their own, so they adopted girls or castrated boys.
Researchers in South Korea analysed the genealogical record of these "eunuch families".
They worked out the lifespans of 81 eunuchs born between 1556 and 1861. The average age was 70 years, including three centenarians - the oldest reached 109.
By comparison, men in other families in the noble classes lived into their early 50s. Males in the royal family lasted until they were just 45 on average.
There are no records for women at the time for comparison.
Dr Kyung-Jin Min, from Inha University, told the BBC: "We also thought that different living circumstances or lifestyles of eunuchs can be attributed to the lifespan difference.
"However, except for a few eunuchs, most lived outside the palace and spent time inside the palace only when they were on duty."
Instead he thinks the data "provides compelling evidence that male sex hormone reduces male lifespan".
Men v women
Women tend to outlive men across human societies. However, theories are hard to test in experiments and the exact reason for the difference is uncertain.
One thought is that male sex hormones such as testosterone, which are largely produced in the testes, could be damaging. The researchers said the hormones could weaken the immune system or damage the heart. Castration would prevent most of the hormone from being produced, protecting the body from any damaging effect and prolonging lifespan.
Dr Min said: "It is quite possible that testosterone reduction therapy extends male lifespan, however, we may need to consider the side effects of it, mainly reduction of sex drive in males.
Dr David Clancy, from the University of Lancaster, said: "The results are persuasive, but certainly not conclusive."
He said the relatively high number of centenarians in the group suggested eliminating testosterone may have prolonged life. However, he cautioned that difference in lifestyle could have had a significant impact.
"In this case eunuchs were raised by eunuchs over generations, lifestyle differences may have been reinforced in this way.
"Castrato versus non-castrato singers are probably a better comparison, and showed no difference in lifespan. Non-castrato lived an average 65 years and both groups lived fairly cosseted lives."