If a person commits a crime as a juvenile, but the crime is not reported until the person is an adult, can they be tried as an adult?
In general, the person cannot be tried as an adult. However, that will vary with the nature of the crime.
The important factor here is not the age when the person is charged. It is the age the person is when they commit the crime. This is because the whole reason for having a separate juvenile justice system is the idea that people below a certain age are less responsible for what they do. Therefore, the important thing is the age they are when they commit the crime.
Of course, some crimes committed as a juvenile can be tried in adult court anyway. So, for example, a person committing a murder when aged 17 may well be tried as an adult if they are not caught until they are 18. But that is not because of the age when charged, it is because of the combination of the relatively high age at the time of the crime and the severity of the crime.
They MUST be tried under the Criminal laws AT the time the crime was committed. IF the crime was one of an adult nature by law to be bound over at the time of the crime, then it matters not when the crime was reported or the Corpus Delicti was discovered.
This of course is state specific, but a good bet is all states adhere to these commencement of prosecution facts.
Some years ago, as a correlative example, the US SC struck down a law which revived a once passed Statute of limitations, substituted by a new one, so criminals could be punished uner the new SOL from a once stale crime. This violated the Ex Post Facto Cluse.