Back when I got into the fandom (around 2007) it seemed to be 35-40 but now it's around 30. Pretty ridiculous how it keeps getting lower and lower every year. That said maybe it's a reflection of our modern society. People are expected to mature earlier and earlier and handle stuff they wouldn't have had to handle until they were a lot older 30 or 40 years ago.

You're a greymuzzle once you're older than the person calling you a greymuzzle. :)