Admittedly, I’m not too fond of the idea of a character who is a demon, so this does have cause to give me pause. However, the idea of someone who is 700-years-old pretending to be someone much younger, that I find interesting.
In my own writing I have some characters who are rather old. The oldest at the moment is around 750-years-old, so right around this age range. Due to various events in the story, he’s not really available for me to use yet. There is another character, whom I have used who was at his introduction 725-years-old, give or take a few months depending on when exactly in 1299 AD his birthday was, not exactly a little kid either.
With this character, it appears that the person who made the prompt was talking about posession. The demon is using a man or woman as a disguise. This could be just that they have this appearance, but it sounds more like there is an actual person whom they are using as their disguise.
The question then becomes, what kind of person is this demon? Are they a true demon? Are they some sort of monster whom people call a demon? Do they care about the host body and does the host body have people who care about them, people who may or may not know what is going on? What’s your take on this situation and this character?