I did some very unofficial testing a couple of winters ago. I had several spools of fluoro that were at least 3 years old, and testing them they broke at roughly 50% of their rated strength. As stated above, it had become opaque. Two year old stuff broke about 80%.
Now whenever I buy a spool, I get a sharpie and write the month/year on it so I know exactly how old it is. If you aren't sure, just test it!