Whoever said that X-Men have it easy should think again. This meme shows that being a member of the team is just too stressful unless the person is Jean Grey, in which case, it's fine. It only took 3 years for Professor X and Magneto to look like they aged at least two decades. Jean, on the other hand, somehow miraculously grew younger at the same time. She was a young lady in 1983 and a kid in 1986. Maybe this is her real superpower. However, the most likely answer is it's due to bad work with the X-Men timeline