What happens if a minor (17) commits armed robbery with a bb gun while under the influence of drugs and arrested several days later at 18 years old?

There are a lot of variables in the case you mention. Even though he is 17, he will be charged as an adult at that age in Michigan. Armed robbery is also a very serious offense which carries a possible sentence of life in prison. If he was under the influence of drugs or alcohol that may or may not affect the outcome of his case. If his attorney believes that it does, he could be sent to the forensic center to see if his substance use prevents him from being legally responsible for the crime. That is doubtful, but it is a possibility that should be explored. If he has no prior criminal history, his sentencing guidelines will be low but this is a violent assault crime and even for a first offense can involve a long prison stint. A good outcome would be the charge being reduced to something like unarmed robbery, which is the usual plea offer for armed robbery cases, which would make him eligible for HYTA status. That is a special sentencing option for people who commit certain offenses between the ages of 17 and 21. Upon successful completion, he would have earned a non public record of the offense and can honestly put on applications that he has not been convicted of a crime. Armed robbery is not an eligible offense because it is a life offense. This is where having a good attorney will greatly help him because he may be able to point out problems in the case that could get the charges reduced or get the prosecutor to not object to HYTA status. Consult with experience criminal defense attorney in your area.

In OK the minor would be charged as an adult from age sixteen and up, so the birthday would be irrelevant.

He can be charged with multiple felonies and be committed to the department of youth service until he is 21 years old or he can be bound over and tried as an adult and get sent to adult prison for 5-10 years or more.

He is a juvenile delinquent as he was 17 when the crime was committed.

He gets arrested and he retains a lawyer to handle the case. Being high is not a defense to armed robbery. Get a good lawyer since you are facing a prison term for a violent felony.