1. Charles Xavier (X-Men)
Between X-Men: Days of Future Past and the opening sequence in X-Men: The Last Stand, Charles Xavier goes from sexy bearded early 30s James McAvoy to bald terrifying CGI-de-aging old man Patrick Stewart. I know the stress of running a school might age you a bit, but yikes.
2. Obi-Wan Kenobi (Star Wars)
George Lucas really didn't think through the time difference between Revenge of the Sith and A New Hope very well. Apparently, in less than 20 years, The Force and Jedis as a whole are basically myths that no one remembers, the Empire has completely taken hold of the entire galaxy, and Obi-Wan Kenobi has aged about 40 years. It's not like his life was even that stressful - he lived by himself in a hut in the desert. It's incredible he aged so much while avoiding getting a tan on a planet with two suns.
3. Marty McFly (Back to the Future)
30 years is a long time, but not THAT long. For contrast, here's what 47 year old Michael J. Fox ACTUALLY looked like:
4. Benjamin Linus (Lost)
Part of the challenge that came with Lost doing flashbacks was they often were forced to use the same actor playing well below their actual age, because it's too hard to find someone who looks EXACTLY like a younger version of Ben Linus and also can act. While this worked fine for the most part, you ended up with a early-50s actor playing a 28 year old, and it showed.
5. Walter Donovan (Indiana Jones)
What a bummer! Right when this guy achieves his goal and finds the holy grail, he starts aging like crazy. Bad luck, I suppose.