Stack Overflow was in private beta up to September 15th, 2008. The first month of public operation ended October 15th, 2008. Even then, the site was still in beta and under significant heavy development.
Even though the community user was created on July 31st, it seems fair to say that all the users involved in the private beta are "old hat" and even those in the first month of public beta.
However, I hate corner cases, and creating a special algorithm change just for one site is annoying. So consider changing the community user's join date to September 15th, 2008, the day the site became public knowledge.
This should allow all those who joined the site prior to October 15th, 2008 to enjoy the privilege of being recognized as being old hat.
Given that this hat season is in full swing, I understand declining to make the change for this year, but please consider it for next year.
Or add a corner case for SO, since it's clearly quite special, if altering the join date of the community user is unthinkable.