Is it legal if someone has been charged at 17 but sentenced at 18?

Yes. Someone can be arrested at any age and be sentenced a year or so later. The question is what law applies (Juvenile or adults). That is a determination for the Court to make at its own discretion following the guidelines in the Florida Statutes and Florida case law and of course the U.S. Supreme Court decisions and the Constitutions of the United States and Florida. Generally a 17 year old will be treated like an adult on most serious law violations. However, they are still eligible for Youthful Offender sanctions which can waive minimum mandatory prison sentences and reduce their exposure to prison time.