I believe it means 16 & 17 year-olds can be held as juvies until 25, instead of being released at 21. It means they can be held longer as juveniles for serious crimes, in exchange for not being put in the adult system automatically.
18 year-olds and up would still be tried as adults.
When I worked with juvenile delinquents, they got turned loose on their 18th birthday, no matter their crime or sentence with a clean record.