According to the theory of special relativity proposed by Albert Einstein, time dilation occurs when an object moves relative to another object at a significant fraction of the speed of light. Time dilation means that time appears to pass more slowly for the moving object compared to a stationary observer.
In the scenario you described, where a person travels at close to the speed of light and then turns around, both the person and a stationary observer on Earth would experience time differently. From the perspective of the person traveling at high speed, time would appear to pass slower for the person compared to the observer on Earth.
When the person turns around and returns to Earth, they would find that less time has passed for them compared to the time experienced by the observer on Earth. This effect is known as the twin paradox, where one twin (the traveler) experiences less elapsed time than the other twin (the stationary observer).
Therefore, from the perspective of the person who traveled at high speed, they would indeed be "younger" relative to the observer who remained on Earth. However, it's important to note that the effects of time dilation due to high-speed travel are only significant at speeds close to the speed of light, which are currently beyond our technological capabilities.