It's not a big problem if everyone is civil and existing moderation mechanisms aren't overwhelmed; people quickly learn from online faux pas and the online social norm is restored.
Also, on pseudonymous sites, you may not even be able to know this at a glance. Sometimes on reddit I have been baffled at the replies I've received, until I realized it was coming from a child, or an older conservative person living on disability.
It's not that I was mean or nasty, or pushing any boundaries – I was never like that – I was just ... 15.
Also turns out what people meant with "your English is very good" was "your English is very good for a 15 years old non-native speaker", and not "your English is very good".
This is an argument against migration then.