Ah, found a post on the newbie whitelist request thread that suggests a script runs at intervals to upgrade qualifying newbies to to Jr. Member. The script runs approximately every 10 minutes, so you have to wait a little after post #5; you don't get upgraded immediately after post #5.

I wanted to edit my original post above, but couldn't find a way to do it (sorry of that means I'm a newbie to forums in general, which I am)