Tavis PoV: "A horde hadn’t been seen in more than fifteen years—not since before Tavi was born,"
Gaius: "The furies of earth and air and wood whisper everywhere that something dangerous is abroad and that the peace our land has enjoyed these past fifteen years nears its end."
Amara: "Marat? The savages hadn’t given the Realm any trouble since the incident on this very site, fifteen or sixteen years ago. Amara had"
Amara: "The Marat had returned to the Calderon Valley, something that had not happened in nearly seventeen years."
Isana about Araris: She turned to face him and said, very quietly, very firmly, “Yes, Tavi. He died. Fifteen years ago. Do you understand?”
I took those from an e-book, so i don't have page number either...
So, accounts vary from 15 to "nearly 17" years
Tavi looks young for his age, Amara estimates him at 12-13.
Isana might have been able to disguise Tavis age from strangers, but there is just no way that she'd be able to shave 2 years of his real age when dealing with people she sees every day.
What would she say: "Oh, this child here, my nephew... he was born two years after his mother died" ??
No, people knew how old Tavi was, Isana just made him look younger so that people wouldn't get ideas.
I think 15 Years is pretty well established, but that my be just me.