I dont think FTL *causes* premature aging, but rather, like in real life, due to time dilation of travelling at high speeds, time passes differently during space travel (I'm sure you've heard the 'old example of "if you sent an astronaut to Mars for a 1 year round trip, upon returning to Earth x-amount of years would have passed") it's a real thing but it works both ways depending on the type of dilation (for example, gravitational dilation caused by large astrological bodies or black holes etc can make one appear to have aged faster) in reality, nobody is aging any faster than anyone else, it simply appears that way due to time passing at different rates. Cryotubes freeze a passenger at point of departure, therefore, you would expect that no matter how time passes differently, they would appear exactly the same as they left upon arrival once thawed. 😊👍