"It is" always = "det er". It doesn't conjugate to a noun. E.g.: "it is a man" = "det er en mand" and "it is a child" = "det er et barn". If you say "that noun" then you conjugate it. E.g.: "that man is blonde" = "den mand er blond" and "that child is young" = "det barn er ungt"
It is because the three make out a language continuum and as such you could clame they are all just dialects of the same language. There is a saying "A language is a dialect with a navy and an army"
PS: As a Dane I really don't wan't to imply that Norweegian is the same language as Danish though... Because we kind of did something bad...