Joshua Phillips was tried as an adult last month and convicted of first-degree murder in the Nov. 3, 1998 death of Maddie Clifton, who lived 25 feet across the street from him in Jacksonville.
Phillips had even pretended to help in the neighborhood search for Maddie when there was hope she was still alive.
The sentencing came after emotional pleas from the Phillips family, and from the Clifton family, in a hearing that lasted about an hour.
Joshua's father, Steve Phillips, said it was "ludicrous and obscene" to prosecute the murder charge.
Maddie's mother, Sheila, talked about a little girl who was effervescent, giggly, loving. "No one or nothing can look up at me with those big brown eyes," she told the judge. "Pictures and memories, that's all I've got."
"I do not perceive you to be a child," said state Circuit Judge Charles Arnold. "Your monstrous act made you an adult."
Because he was 14 at the time of the slaying, Joshua faces a maximum sentence of life in prison. Florida law bars the death penalty for killers under 16.
The trial was moved 400 miles to Polk County in rural central Florida because of intense publicity in Jacksonville.
The waterbed where Maddie's body was hidden for seven days was reconstructed in the courtroom.
CBS affiliate WJXT-TV in Jacksonville reports the defense rested without calling a single witness. Instead, attorney Richard Nichols was allowed, under Florida law, to present two closing arguments sandwiched around the prosecution's close.
Nichols, who has come under fire from Florida lawyers for his handling of the case, had planned to submit a report by a psychiatrist who examined Phillips for the defense. But when the judge ruled the psychiatrist would have to testify in person and submit to cross-examination, Nichols decided against calling him.
Nichols said that while the case might sound like a "horror story like from Stephen King" it was not premeditated murder. He urged a manslaughter conviction instead of first-degree murder.
"This case was open and shut," said prosecutor Harry Shorstein.
Phillips never denied tha he killed his young neighbor. He told police he accidentally hit her in the eye with a baseball as they played in his back yard. He panicked at her screams and was scared his father would punish him.
So he dragged Maddie into his bedroom. He smashed her in the head with a bat to stop her from screaming and when she kept moaning, he grabbed a knife and stabbed her in the throat, he said.
He shoved her under his waterbed and went to wash up. Phillips still heard moaning. He pulled her from the bed and stabbed her until she stopped breathing, detectives said.
An autopsy showed the girl was beaten over the head and stabbed at least nine times in the chest and twice in the neck.
The night she disappeared, Phillips grabbed a flashlight and joined volunteers scouring the neighborhood for Maddie.
Days later, Joshua's mother, Melissa Phillips, discovered Maddie's decomposing body beneath a sheet of plywood supporting the frame of Joshua's waterbed and summoned police.