How old can references or sources in a thesis be?
References can be as old as they need to be to cover the material.  I had some that were more than 30 years old. But if all of your references are "old," people are going to want to know why.   

You must also be sure you cover the most current research in your field.  A few in my own dissertation were for material published in the same year as my own work.

The link given in the comments and the revised question seems to be directed toward undergraduate research assignments, and the "ten years" reference is a part of an example assignment, not a requirement given by the Shapiro Library. The key idea in the link is that references must be "somewhat current." 

For a thesis or dissertation, one must cover the field, including both early and very new research.