Hello! I'm potential apper and I have question about the age-limit!

Would a character whose physical appearance/age differs from their real age? To be more specific: I'm thinking of a vampire who has been around over 200 years but has an appearance of 14 year old boy. The character is mentally more mature than normal teenager and capable of handling dark and gruesome events (well, he is a vampire) but also has tendency to act like childlike..? So uh, I suppose it could be said that he's in weird limbo of being both adult and child? Idk, if this makes sense.