His hair

Are you kidding? Think for about 1/2 second next time. Here is a clue.....look at a photo of the real Patton.

Because George C. Scott, in his early forties, was playing George S. Patton, who had been in his late fifties at the start of WWII. Patton had a receding hairline. You'll also notice they whitened Scott's hair for the movie as well, if you compare it to his appearance in other films made around the same time.

Really, you're question is akin to asking why Daniel Day Lewis sports a beard in "Lincoln" when he doesn't wear one in real life.