Is a year old frozen turkey still ok to cook?

Well, you can certainly try. If the turkey was commercially frozen from the start, and has remained frozen ever since, it might be safe from freezer burn. If it was fresh when you bought it and you froze it, or its been defrosted at any point, the results could be more dicey. But generally speaking, unless they were commercially frozen, or you used a high end vacuum sealer, you shouldn't leave meat in the freezer more than a couple of months.