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

Writing in short form where a lengthier reply would work better and when confronted with such a reply some form of "not reading all that" or other thought terminating clichés.

I've found that as arbitrary as it is - the ability to read a lot of information works pretty well as a mark of mental maturity. Also links to Twitter or Tiktok as discussion points. No one over the age of 19 browses TikTok lol.