I imagine that part of it comes down to motivation. I pretended to be an adult on a special-interest forum when I was twelve years old because I needed an escape from my miserable existence. At that time, I had no control over my life and every morning I woke up meant I had a new chance for traumatic shit to happen. I desperately needed to be someone else, so I took my time, researched shit, and avoided any conversation where I might be outed. I'm sure I didn't fool everyone, but I got some shocked responses when I went back as an adult and owned up to it.
Kids doing it for the authority boost or just as a childish fancy will be easier to spot. Kids doing it as a coping mechanism for their horrible lives will probably blend in a lot better.