Possibly because there might be another word or phrase for grown up?? It's like mother and mum, we know that they are the same thing but we use different words. Maybe that's the same for adult and grown up.
5 years later and still no answer as to why "Now you are an adult" is wrong. DL needs a module on sentence construction and word placement. Yeah, I know, it is free and I should look somewhere else for this.