You people must have really big freezers. We're forever misplacing this leftover or that packet of yeast or some other small thing in the freezer. But how in the world do you lose track of a turkey for years on end? We were going to wait to answer until someone asked about a 5-year-old turkey, but apparently those people are just too embarrassed to ask (we know you're out there).
The surprising answer to all of you is that in terms of safety, frozen turkeys will keep indefinitely in the freezer. Quality is another matter, however, and is said to decline after about a year. So only one of you is assured of having a juicy, flavorful turkey that you could confidently serve to family and guests (assuming you don't overcook it).
The others might be fine, as well. But, at least for the seriously aged birds, we would not cook them with the intention of impressing our future in-laws at a holiday banquet. We'd thaw them, see if they looked satisfactory in their raw state, then cook them and, again, pending a satisfactory taste test, make buckets of soup, casseroles, and all sorts of other dishes that could then get lost in the newly spacious freezer.