I think a lot of Michael Barnett's case hinges on her being diagnosed with mental health issues which she wouldn't have been diagnosed with had she been a child, therefore she must be an adult. But as far as I am aware, there's no single set of symptoms which occur only in adults and not in children - the diagnosis of those symptoms would merely vary depending on the age of the patient. If the Barnetts had placed N in psychiatric care and been able to convince the doctors that they believed N to be an adult, the doctors would treat her as such and thus her diagnosis would be an adult one. It's a classic Munchausen by Proxy move - get a diagnosis based upon a faulty parental history and from that diagnosis extrapolate evidence, coming back to prove the original diagnosis.
I imagine turning up in court with a box of the girl's baby teeth would be fairly compelling, but people have been acquitted with more concrete evidence than that against them. I hope the state have done their work thoroughly - because if the Barnetts *are* guilty, what's been done to this child is shameful.