How quickly do you age in space?

According to the theory of relativity, a spacecraft's clock does not keep the same pace as an atomic clock on Earth. However, according to ESA, the effect is very small: typically differences on the order of milliseconds after several months. To precisely determine the orbit of space probes this must be accounted for, but for a person's biography it means nothing.

The interesting part is biological. According to NASA data, weight-bearing bones lose on average between 1 and 1.5% of their mineral density per month, muscle mass is lost faster than on Earth, cardiac performance can decline, body fluids shifted toward the head can press on the eyes and affect vision, and the immune system is altered. This pattern resembles aging. NASA's Twins Study with Scott and Mark Kelly showed that these changes reach even gene regulation: spaceflight intensifies methylation, the mechanism that turns genes on and off.

What matters is the difference between adaptation and damage. Much reverses after landing, but even rehabilitation doesn't fully compensate for bone loss. What is not reversible is the radiation dose: radiation is associated with a higher risk of cancer and conditions such as heart problems and cataracts. So someone living in orbit doesn't age faster in calendar years, but they do take home a risk profile that gets carefully assessed.