Sentencing can certainly carry over and yes, the person will be charged as an adult for absconding if the crime continued until after the person turned 18. It is hard to say how much time the person may serve without knowing more about the underlying juvenile offense. The underlying juvenile offense is not re-tried. The person should hire an attorney to surrender to the court on a bench or arrest warrant, which surely is pending.
The person will be charged as an adult for absconding and the prior sentence will carry over. An attorney should be retained to deal with this.
ANDREW ROBERTS CRIMINAL AND TRAFFIC TICKET DEFENSE ATTORNEY
I concur with my colleagues, yes, this could carry over and a warrant and good amount of jail time and additional charges are certainly pending. This person needs to hire a good, local criminal defense attorney asap, to coordinate getting himself turned in, etc. This is serious.
When they turn themselves in at 18, they can be charged as an adult for absconding from the facility. The sentence can be carried over, so the best advise is for that person to contact an experienced attorney.