If a person committed a crime as a juvenile but the crime wasn't reported until they were an adult, could they be tried as an adult?

From the facts that you have written, it is entirely possible for your nephew who committed a crime when he was a juvenile to be charged as an adult. The issue that I see is whether the alleged crime is barred by the applicable statute of limitations.

Assuming your nephew is convicted of the crimes that he is charged with, there is the possibility that he can be sentenced to jail. I suggest that he consult with a criminal defense attorney to defend him and to look into possible defenses under the applicable statute of limitations.