The 4th District Court of Appeal in West Palm Beach ruled that it was not established before or after the trial that Lionel Tate, was competent to stand trial.
"We felt that both pretrial and post trial there needed to be a finding of competency before this child could be tried as an adult," attorney Richard Rosenbaum told CNN.
No trial date has been set.
Tate was convicted of first-degree murder and is serving a sentence of life in prison, a term that child advocates say is too stiff for a child.
Tate has said he had only given Tiffany Eunick a bear hug. Prosecutors said he beat, kicked and stomped her over a five-minute period.