I usually go to Special:Contact and let Fandom Staff know. They then block the user for however long they see fit.

I usually block them depending on how old they say they are. For example, if they're 11, I just ban them for 2 years

It should be noted that officially, Fandom staff don't request admins to do anything about it, neither block or report. However, admins are free to block for almost any reason they want.

If you know they’re age, you can ban them until they’re 13.

Or you can be cruel and report it to FANDOM. The account will be disabled as a whole, and they cannot log into the account ever again.

Just FYI, 13 is not the only age you need to be aware of if you are concerned about underaged users. Anyone in a country that is a member of the European Economic Area (essentially the European Union) or the US state of California needs to be 16 or older.

I haven’t experienced anything like it yet, but I guess I would only ban them until they turn the right age.

I have seen in many wikis that the admins will block them forever due to them revealing age for their safety.