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

Excessive edginess

It's important to differentiate it from like. Having extreme opinions. There are plenty of adults with extreme opinions and they are a whole other conversation. -- But only people under a certain age (not strictly kid, mind, though most people have shaken this off by their mid 20s) have a penchant to arrive at extreme opinions specifically because they are "edgy" and "cool".

This tends to also come with a particularly needless hostile attitude, where they very quickly and easily start with the verbal abuse.