Not necessary!!!
We can see in many stories that humans hate demons, half beasts, etc... or are scared of them because they are different!
In fact this is a real thing in humans, seeing that racism still exist!
So most probably they saw the castle, knew there was a new Demon King and be because they are scared or because they hate have demons living near them, they began to exagerate or lie in order to get a hero kill the demon king and the demons!
Even in mangas where the demons are the bad and is seen from human perspective, we can see how humans hate other races because they are different, and himans tent to believe they are superior even though they are superior to other races only because their numbers and not abilities!
would be interesting to see the faces of the shit stirrers when the hero brings down the demon lord for a shopping day,
"did you defeat the demon lord?"
"you're looking at him"
"but he's a baby?"
"well he did only appear a month ago what were you expecting?"
lol
what i want to know is who has been impersonating the demon lord and spreading enough chaos that a hero was sent to assassinate them? Cause the stories of a demon lord has to start somewhere and i doubt a baby could do that, the only other possibility is some kind of psychic or oracle could see the demon lords potential but got the time wrong .