U.S. Magistrate Judge Andrew J. Peck said Abdiwali Abdiqadir Muse could be treated as an adult in U.S. courts after a closed hearing during which he said Muse's father gave conflicting testimony about the ages of his children.
Muse was charged with several counts, including piracy under the law of nations. That charge carries a mandatory penalty of life in prison.
In addition to piracy, he was charged with conspiracy to seize a ship by force; discharging a firearm; aiding and abetting the discharge of a firearm during a conspiracy to seize a ship by force; conspiracy to commit hostage taking; and brandishing a firearm.
Peck noted that Muse had declined to testify in court about his age.
The piracy charge falls under an obscure federal law dating to 1819 - 60 years before the invention of the light bulb, reports CBS News chief investigative correspondent Armen Keteyian But as pirate attacks have surged - 102 and counting already this year - bringing pirates to justice has proven to be anything but easy.
"Under the laws of the sea, any country can prosecute piracy," said naval investigator Mike Sliwa. "It just becomes a challenge to identify which country wants to and can accept them for prosecution."
Witness last weekend - after a group of pirates attacked a Norwegian tanker off the coast of Somalia, the Canadian Navy spent seven hours hunting the pirates down - only to release them because the Canadians can only arrest pirates who attack Canadian ships, Keteyian reports.
Assistant U.S. Attorney Brendan McGuire said Muse initially told a Somali interpreter on April 12, when he was first detained, that he was 16, then that he was 19, then that he was 26.
Muse indicated a day later on a different U.S. Navy vessel that he was 19, McGuire said. The prosecutor said Muse told an FBI agent Monday that his age was 15 but later apologized to the agent for lying, telling him he was 18, going on 19.
McGuire said investigators also spoke to one of Muse's brothers, who indicated he was 18.
Muse is the sole surviving Somali pirate from the hostage-taking of an American ship captain from Vermont.
He arrived in New York on Monday evening, handcuffed with a chain wrapped around his waist and about a dozen federal agents surrounding him.
His left hand is heavily bandaged from a wound he suffered during the skirmish on the cargo ship, the Norfolk, Virginia-based Maersk Alabama. Muse, his 5-foot-2 frame so slight that his prison clothes draped loosely, at one point put his head in his uninjured hand.
When the judge asked him if he understood that court-appointed lawyers would represent him, the teenager responded through a translator: "I understand. I don't have any money."
When he was asked to raise his right hand, he pointed it into the air as if he was being called on in class.
Muse arrived in New York on Monday night and smiled as he was led into a government building.
"The last time I saw him he was in his school uniform," Muse's mother, Adar Abdirahman Hassan, 40, told The Associated Press by telephone Tuesday from her home in the central Somali town of Galkayo. People who are older than him outwitted him, people who are older than him duped him."
She said he was "wise beyond his age" - a child who ignored other boys his age who tried to tease him and got lost in books instead.
"He took all his books the day he disappeared, except one, I think, and did not come back," she said, adding that she did not know which book he was reading - Hassan is illiterate.