Where did you get the time was only 18 years? But as to Obi-Wan looking so old, being in hiding, a shut in, plus also dealing with the fact that your being hunted and the fact all your jedi brethren are dead, plus stress will make anyone look old.
He got the "18 years" thing from the fact the RotS takes place 19 BBY.
Short answer for you OP is that stress of being hunted by the empire and the intensity from Tattooine's two suns rapidly increased the aging in Obi-Wans face and hair.
And to your other question: The amount of Imperial propaganda was extremely high in the Dark times. Many people eventually came to believe that the Force actually WAS just superstitious nonsense.