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I could be wrong about this (don't you just love answers that start this way), but I don't think Jenny and Jake are "official" terms. I think they're more farm terms, and are somewhat regional, commonly used in England and some areas of the U.S., but not used at all in other areas. Kind of like the term "rooster." Everybody uses it, and everybody knows it means a male chicken, some people even feel that it means a mature male chicken, but it's not an "official" term. And don't even consider using the term "roo" around high-end breeders -- many of them won't think twice about correcting you!
But you got my curiosity going, so I got out the books. The American Standard of Perfection, 1998 edition, lists standard weights for an "Old Tom," "Old Hen," "Young Tom," and a "Young Hen." I could not find the terms Jake or Jenny anywhere. I also could not find a cut-off point where a hen or tom goes from being "young" to "old," unlike with chickens where the transition from pullet to hen or cockerel to cock occurs at 1 year of age. With turkeys being larger birds and (hopefully) growing a bit slower, I assumed that they went from young to old at sometime later than 1 year of age, but don't know for sure. (Anyone on this thread show their turkeys? Do you know what the official transition age is?)
I also looked in the 6th edition of British Poultry Standards (2008). There it lists standards weights for a "Mature Stag," "Mature Hen," "Young Stag," and "Young Hen." Again, no documentation as to when a bird transitions from "young" to "old," and no reference to the term Jake or Jenny.