it's the hardest, most stressful job in the nation, if not the world

[graham4anything]

and also the natural aging process
(only Reagan kept his jet black hair, but we all know that was not his natural color, as people who saw him after his retirement without hair dye instantly noticed.)

They all have aged in the four years, far greater than four years.
Jimmy Carter, Bill Clinton, even Nixon (who always looked older than he was) aged greatly.