What happens if a minor commits a crime but then prosecuted as an adult..... are they charged when they committed it?

I don't understand your question

The determination to charge a minor as an adult does not come at the time they are arrested, it comes when the District Attorney reviews the case.

In my state, once a minor is charged as an adult, all future charges are automatically charged as an adult. So, if they are arrested again the officer could treat them as an adult at the time of arrest.

Mevwe

The DA decides how to prosecute based on the severity of the crime. A juvenile will either spend his time until he reaches the age of majority and is then transferred to an adult facility or is isolated within the adult facility until he is an adult.

A crime committed by a child cannot be tried in court as an adult. When the child becomes an adult, the crime must be tried as a child offense. That way, the sentence is given as it would have been at the time the offense was committed.