What happens if a minor commits a crime and then charged when an adult?

They'll be charged as an adult. There's cases where the trial has been postponed until the defendent has reached adulthood so that they can be tried as an adult, normally only when they're close to being an adult anyway and the crime is a bad one/multiple offences.

What happens depends on BOTH the jurisdiction, AND the crime. In some cases, they couldn't be prosecuted after 4 years, even if they were a legal adult at the time of the crime. In others, they would still be tried and sentenced as if they were a juvenile, and they could be held by the juvenile system up to age 25. In still others, they could have been tried as an adult at 14, and will still be tried as such.

The charge is as if he is a minor.

If convicted he/she Will receive adult punishment!

He could be charged as a minor or an adult, depending on the laws where it happened.

Law and Regulation Questions 

Laws are different depending on where you live. When asking questions about law, you need to state the applicable city, county, state, and/or country. If a regulation you may need to give program name. 

Please update your question and not in a comment to my answer, so others will see it. 

Criminal prosecutions and sentences depend on many things. The location of the action, your age, the charges pursued by the prosecutor, the laws applicable and penalties prescribed in the law in your location, the judge involved, the defense attorney, the amount of discretion the judge has, etc. In many cases people can’t begin to guess an outcome unless they know really specific details.

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