Can I safely freeze turkey stock I made 6 days ago?

"a fine tack"

Freezing will suspend the stock's current microbial state which is probably fine but technically suspect at this point. Boiling will reset the clock so to speak.

Six days is past the recommended (refrigerator) storage time. The USDA recommends that you discard it. If it were me, however, I'd bring the stock to a boil for 10 minutes then cool in an ice bath before freezing. There are those who will argue against doing so and I think that's fine tack, too. It's hard to go wrong following USDA recommendations.

Here is what Harold McGee has to say about the situation:

http://www.nytimes.com/2011/08/24/dining/bending-the-rules-on-bacteria-and-food-safety.html?pagewanted=all