A user under the required age (usually 13) to join an online platform.
Ex.

Bob is 11. Bob joins a 13+ platform illegally. Mandy, who is 16 and is a staff member finds out and bans Bob for being an underage user.
by Henzo1 December 14, 2021
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A person who is under a set age. "Underage User" may be used for forums, online games ect... Anywhere online where there is a set age you must be.
Ex.
Bobby is 12. Bobby joins a 13+ forum. Mandy who is 14 and a staff member finds out- Mandy banns Bobby's arse =) and did it because he was a Underage User
by Ryaa July 18, 2006
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To be yelled at anyone on forums who says something immature or stupid, suggesting a preteen poster.
Iamover18 : Liek Teletubbies r teh Rox
Sinner: UNDERAGE USER ALERT!!!!!!!!
by Kissingtheshadows March 25, 2005
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