My personal bet is that Tavi is younger then it is claimed.

In the chapter one of Cursor's Fury (just recently posted), it again covers the same ground of Tavi coming into things later then normal. In this case he had his growth spurt later then expected, now he is Max's height.

The other reason I believe he is younger then he is claimed to be is the lack of furies.... Now it is possible he will never have furies.... Only Jim knows for sure. But if he does get them, one possible reason is he is a late bloomer, wihs I had book 2 with me, but there was a comment in there about which age a late bloomer attained their furries and I suspect Tavi is right around that age.

But I am rehashing old ground.