Time dilation

Time dilation is a physical effect first theorized by Albert Einstein in the 20th century. Essentially, when an object accelerates closer to the speed of light, the object experiences less time passing than an outside observer. Speeds that produce noticeable time dilation are referred to as relativistic speeds. In the 24th and 25th centuries, most federation citizens never experienced the effect, since warp engines don't produce a time dilation effect. 

In the 22nd century, during the start of the Earth-Romulan War, the starship Columbia (NX-02) was the subject of a Romulan experimental attack. As a result, the ship's Warp drive was disabled. Captain Erika Hernandez ordered the ship to travel toward the nearest inhabited planet at relativistic speeds, a journey which would tzke 6 months from the perspective of the crew, but years to the outside universe. 

In 2409, Charles Ryerson was discovered near Barnard's Star by the USS Zife, having been traveling in his small craft at relativistic speeds for 20 years. As a result, he had aged very little over that period of time.