Bone fusion is not an exact measurement, it can vary widely in individuals. This is a X-ray analysis we are talking about, it is not as precise as direct examination of the bones themselves, which obviously you can't do when they are still being used.
The first bone fusions start anywhere from 9 to 15 years of age in normal individuals, some of the later ones can take as long as 30 years. Plus, we are dealing with a dwarf here, who by definition is an abnormal individual. The doctors who made that assessment were likely used to dealing with normal children with normal development. They may say 8 or 10 using the lower end as a baseline, but it could just as easily have been 15 or 18 at the high end, maybe even more if the individual was outside the normal range.
It might make for good reading in an affidavit to get a warrant, but in reality what they are describing is a lot more vague than that. And remember, this information is from an affidavit citing records, but there is no reason to think that those original examinations would have been done objectively and with critical review, there was no reason to have done that at the time. The examiner just would have just done their X-rays and used the usual criteria based on normal average children that they typically looked at. Now, those results are actually important for the current case, but will they stand up to critical analysis? The LEO filing the affidavit certainly had no incentive to do that. You can't just take these things at face value. We will have to wait and see.