Astronaut Scott Kelly returned from a year-long sojourn in space in June. His slightly older 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: Gizmodo – Why Mark Kelly Is Now Older Than His Older Twin Brother