It does vary by the circumstance, by the crime, and by the state....Last I heard committing a crime at 14, 15 or 16 does not mean freedom of consequences for the crime.
My sister's first job out of college, (which was mid school year, because she graduated a semester early) as a Special Education teacher was at a juvenile detention center where they are held, behind bars for their crimes until 18...at 18 there was some kind of process that could take place that could move them to an adult prison, if memory serves...for violent crimes like murder...
There was some kind of point system to earn points for good behavior, for them to earn a release to go and stay at home with their parents or grandparents for a weekend every couple of months or so...as part of their reform. (this wasn't for the extremely violent juveniles there).
She had only one student that was a murderer...he murdered his father who had just beaten his mother to a pulp.