I don't think there has really been any indication that he has been lied to about his age. The first book tells us he is 15, and the second, two years later, 17. at that age, it's smack dab right where it should be for the readers and a few other characters to figure certain things out.
He couldn't be too much younger (a few months, maybe) than the book has told us. So why fudge it there? "muhahah, i have told everyone he is 15 and three months old, but really, he's 15 and five months old! No one will see through my cunning plan to disguise his parentage by moving his birthday"...to a day that still doesn't hide his parentage.
And he can't be too much older, either, or he would have had to be REALLY small for his age, and not just a late bloomer. Therefore, there is no reason for us to doubt his age. I can't believe this has gone on as long as it has.
Er. Or a year or two before, which was my point. The problem is if his birthdate is -after- then he really, /really/ is small for his age. Remember, he's supposed to be small for his age. If his fake birthdate is a while -after- First Calderon, then he's actually -very- small for his age.
There is no need for Isana to lie about his age. It was revealed in the second book that all unmarried women had to serve in the camps of the legions, so I can't believe that Tavi is the only one who was fathered during that time.
Moreover not many of those who might know about Septimus' and Isana's relationship survived the FC and in case someone remembered Isana made up the story of Tavi being the son of her sister.
No need, but if you were hiding something that big, wouldn't you like a little extra 'insurance'? I mean, no one is going to say, "Wow, this furyless kid was born x months after FC, and Septimus was unmarried, he must be the missing heir!" But changing his birthdate might be enough to distract someone who was potentially putting the pieces together. Kind of a Jedi mind trick sort of strategy.