While it may not "prove" the girls' ages, it will certainly give credence to Jaycee's testimony. The fact of the matter is that PG/NG implicitly (if not explicitly) accepted the ages as listed on the birth certificates. Prosecutors will likely call psychological experts to the stand to attest to the amount of control the Garridos had over Jaycee, control that would have made it impossible for her to totally override them when it came to raising the girls. If she was not allowed to tell them that she was their mother, then it follows that she was not allowed to lie to them about their ages. Like I mentioned in my previous post, PG did not correct the girls when they said that they were fifteen and eleven, strongly implying that they were told those ages by PG/NG as well. You wrote that PG/NG would have had a reason to lie about this, but why would they lie and say that the girls were fifteen and eleven when it would have been in their best interest to tell them they're younger than they actually are? IMO, the jury will have no reason to doubt the girls' ages as listed on the birth certificates since all parties (Jaycee, PG, and NG) seem to have been in agreement on this during Jaycee's captivity.
And like songline noted, there are other tests -- e.g., physiological and dental tests -- that can be done to corroborate Jaycee's testimony regarding the girls' ages. Not only that, but judging from the photographs I've seen of their property, PG was a major hoarder. It seems likely that there'd be photographs with dates on them, receipts, etc. Even the manufacture and/or purchase dates of baby/child clothing and toys could be used to construct a timeline. We all know Jaycee kept a journal, and IIRC, forensic scientists can even run tests to determine the age of the ink.
All that being said, I think it will come down to common sense. I understand that you often take the devil's advocate approach, but do you honestly believe that the dates on the birth certificates aren't correct and that Jaycee would have any reason to lie about this? I doubt that anyone here, including you, really believe they're younger than they are or that Jaycee would lie. IMO, the jury will likely feel the same way.