I've read the lemmy.world responses, and they're all predictably trash. The real answer is there's no good way to tell if someone's a kid because they're plenty of mature 12 year olds and plenty of dipshit adults with poor writing skills (ie boomers). To rephrase the question, what is a dead giveaway that someone is actually not a kid?

1. They talk about mid- to low-tier shows that your average zoomer wouldn't know about. So someone talking about The Simpsons or Batman: The Animated Series doesn't mean much, but someone talking about Biker Mice from Mars or Extreme Dinosaurs means they're some millennial.

2. They namedrop social media sites that no longer exist. Remember Xanga and Vine?

3. They complain about body problems. Knees that feel funny before it rains. Bad back. Nose hair that never stops growing. And so on.

4. They demonstrate an understanding on how old and obsolete technology works (typewriters, payphones, VCR, floppy disks, Limewire, DOS, modems, Blackberries). Like, average zoomers don't know how to use those map guides that are in the form of a book.

5. They remember how the pre-modern web was like or life before the web was like. Someone who's <18 years old was born after 2006, when the modern web was still being build.

6. They remember ancient memes and fads that are obscure enough that it isn't some zoomer looking up random memes on knowyourmeme.

7. They're horribly out of touch with what kids enjoy. Notice this is different from passing judgment on what kids enjoy because they're plenty of things I thought was stupid when I was a kid. But when you say shit like "only 13 year olds like Tiktok" or "only kids like Minecraft" yeah you're just out of touch.