Another way can be through roblox links if they uses roblox. If it's a 13+ player in roblox, the link will have www like https://www.roblox.com/ this. If the player is under 13, the www will be replaced with web. They can still uses mistake as an excuse though.
@Andlksjqkkqk Even if they reveal themselves to be underage on Roblox, it's not Fandom's issue. As they need to state it on Fandom itself, not on a third-party platform such as Roblox, YouTube, Steam etc.