I think I know! They have to have opened forums on their app before. That seems to trigger the forums account "creation" and thus allows them to be tagged! I used to think it required at least one forum post but I have seen people tagged that have none, so I think I've narrowed it down to just opening forums at least once.
I don't think it is account age, because I've come across some accounts with 90 day awards that cannot be tagged. It is plausible that if it is triggered by the account age, it is higher than 90 day; however I've also been able to tag ones under that threshold too.