The OChef site states that the quality of the turkey declines after a year, but still usable, say for soups. The USDA website states that the turkey is still cookable, although quality may be an issue. But freezing a turkey no longer than 12 months.
So from that information, go ahead and thaw the turkey, check the quality and then all you can do is to roast it. You will notice if something is not right.
Check out the Freezing Foods Guide on the USDA site -
Freezing and Food Safety
Scroll down to the bottom, where it says, "freezer storage chart". Good luck to you!