Do you have two versions of Pages on your Mac? Check to see if you have a folder named iWork 09 which contains Pages, Numbers, and Keynote. Also check to see if you have a Pages app version 5.5 on your machine.
That's OK. The point is that you need to set the correct default for Pages to the newer version so that your older Pages documents will be converted when you open them. Also, go to Software Update from your Apple Menu and update Pages to version 5.5.1. While you're at it, also update Numbers and Keynote.