I don't think there is a "dead giveaway". Plenty of kids can pass as adults online and plenty of adults seem like kids online. And sometimes with stuff like word usage/grammar/etc you can't tell if it's a child or someone who doesn't speak English very well or maybe an English-speaking adult who happens to type like that. There's a lot of different people in the world.
Yeah seriously, every time someone makes a generalization online "that subreddit is all 12 year olds anyway", "r/teenagers is mainly grown me", it really bothers me because no, you're just overconfident in estimating people's ages from text
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.
I can't get over ironically using stupid lingo, without being good at presenting it as ironic use...so I often seem like a child. I am certainly bad at forming sentences that are not stream of thought (with weird punctuation like parentheses containing clarification...like this...and overused ellipsis...)
It's interesting to meet someone else who also struggles with an overuse of parentheses and ellipses (I didn't know what they were called, thanks for that!).
This is a complete shot in the dark, but do you also happen to be on the spectrum? (I have nothing to base this on expect my theory that overclarification could be more common among neurodivergent people)
I don't present as on the spectrum, but I was diagnosed with ADHD when I was a kid. I haven't suffered from it much as an adult. I have heard that the parenthetical over clarification and typing as your thoughts would flow naturally is a sign of Autism in particular.
I can control it when I focus, but if I am ranting it comes out in force.