I haven't read this book either, but I wouldn't put much credit in the idea that Jesse lived to an old age. I say this simply because the same story is told of every famous outlaw. Billy the Kid, John Dillinger, Butch Cassidy, Emiliano Zapata...it's always the same: they faked their own deaths and lived to old age in obscurity. For Jesse in particular this is true. He was extremely vain and protective of his reputation. He would never have let it be thought that he was shot from behind by a coward. Maybe if he could have faked being killed in a wild shootout against overwhelming odds, but not that.