On the Internet, what is a dead giveaway that someone is actually a kid?

https://lemmy.ml/post/17542340

On the Internet, what is a dead giveaway that someone is actually a kid? - Lemmy

Poor grammar/orthography
This doesn’t seem to hold true for native English speakers. The number of old white North Americans on Facebook who haven’t figured out punctuation, capitalization, or things like their/there/they’re is astounding.
Rest assured it’s not just US problem. The same happens in other languages too and it may be even worse there.