Er, no, you don't get my point. What I was saying is this: do the books -say- outright that Tavi's birthdate (the one Isana gave him) is the same time as the First Calderon? Let's say FC happened 15 years ago. If Isana wanted to hide his age, she could say he's older than that, or younger. He looks 'young for his age' so older than that is possible. It would all depend on whether or not his birthdate is flagged as the time of FC ICly or not.
Though oddly, looking at the book (to get information to comment further on), Tavi insists that First Calderon happened far before he was born--it's in the section with the Princeps' Monument.
Somehow I missed a page of replies discussing this same point, sorry. But for a few specific details:
I looked a the monument section and the next section with Amara, Tavi & Bernard walking to Bernardholt and found a time check - "The Marat had returned to the Calderon Valley, something that had not happened in nearly seventeen years." near the beginning of chapter 15.
Elsewhere in the book Tavi says that he is 15 but others think he looks younger than 15. Amara asks if he's 12 or 13 when he talks about being furyless, and he says that he is 15. 15 plus nine months does not equal nearly 17, so Tavi does not know his correct age.
With Isana confessing to having stunted Tavis growth and specifically saying "I didn't stop" I think she has lied about his birthdate and then stunted him to appear even younger than his supposed 15 years. The main logical flaw to her lie would be the official date of the death of Isana's sister, Tavi's supposed mother, which would be another thing to research. There would be no witnesses other than Isana and Araris to leap out and say "Aha! but the woman was dead a year before you say!" if they went immediately into hiding.