You can seriously imagine someone going from looking like Ewan McGregor to Sir Alec Guinness in the course of 19 years just because of some stress? It is great if you can, but to me that is a pretty massive stretch. Again, nothing personal.
Well, some people really age fast once past 60.
20 years are not nothing on a human body.
Let's say "EP3 Obi" is 40, but looks a bit younger, and "EP4 Obi" is 60, but looks older.
Of course I'm in a full Lucas apologist mode here.