I agree w/ Archer. Maybe Mephy (when he goes out in public) lets himself appear to slowly age w/ his host but uses different hairstyles, etc. to appear like he's aged differently each time. So he's pretended to be his grandfather, father, etc. XD I think it would be possible for him to age himself and then rejuvenate himself, given his title as King of Space and Time.
As for whether Mephisto is the real Mephisto from legend... well, I'm sure something from his past will be revealed eventually. He's always been interested in human culture and aesthetic. That's why his mansion has a European vibe to it. He could have shifted his interest from European to Asian (and more specifically, Japanese) in recent years. It fits w/ original!Mephistopheles interests in humans, after all.
Honestly, I think he is the Mephistopheles from the Faust tale, but doesn't want to reveal too much about himself
