Astronaut Scott Kelly returned from a year-long sojourn in space in June. His slightly older astronaut twin, Mark Kelly, stayed home as a control—part of NASA’s twin study to monitor the effects of space on the human body. But there’s a physical change that NASA might not be able to measure that easily. Mark is now even older (by about 5 milliseconds) than his space-faring twin, thanks to special relativity.
Source: io9 – Why Astronaut Mark Kelly Is Now Even Older Than His Twin Brother