Hey thanks for the reply Natch.
I checked out the Mysql manual on the Now() operand and found it a bit confusing.
UPDATE user SET reputation = reputation + (((NOW()-joindate)*(60*60*24)) / 90 )
This should seed the reputation table with 1 point for every 90 days registered correct? So basically I can run my initial query:
UPDATE user SET reputation = posts/500
Then run your query. I will backup my database and give this a shot tonite! Thanks for the help