If you suspect someone is under 13 and have on-wiki proof of such admissions (such as this user admitting to it, posting their birthday, etc.), please file a report on TSPortal under "Data Protection" and cite as much proof as possible. Off-wiki admissions on places such as Discord or a non-Miraheze wiki cannot be corroborated as legitimate many times and thus cannot be used to cite that a user is under age.
Once reported, you should not spread the word that a user is underage or publicly tag them as such. Trust and Safety will investigate the accusation and action as needed. If we find that the report is veracious, we will lock and vanish the user. Once that occurs, you should still not publicly tag the vanished user as underaged.
Advice to wiki administrators
If you suspect or can prove a user is underaged, please do not locally block them with anything mentioning their age or underaged status as Trust and Safety generally has to return to that wiki to overturn the ban and suppress the log revision. If they become a disruption, you may block with a generic reason such as "Policy violations" but if they aren't a disruption, it's better to wait for Trust and Safety to handle it. Once you can verify a user is underage, please report it to Trust and Safety who will handle it. Make sure to provide as much proof as possible.
Do not report these suspected underaged users on public venues such as the Stewards' noticeboard. Only report this, in private, to Trust and Safety.