Witnesses said it was a typical landing until the emergency door opened and the plane filled with sunlight.
"The whole door goes off. I start to hear some screams," Maria Nauca, from Peru, told Fox affiliate KTVU.
"As soon as we landed, it popped open," Anielka Ruiz, another passenger, told the news station. "We saw the light coming though because all the windows were closed - so the light came in and we're like, 'What happened?'"
Passenger Matt Crowder, from Atherton, Calif., told NBC affiliate KNTV that the teen "jumped out onto the wing and then slipped off" onto the runway.
He said no one was able to stop the boy.
"They were yelling, 'Tell the flight attendants. Relay the message back. The door's open. Someone jumped off,' " Crowder added. "It was pretty crazy."
Copa Airlines said in a statement a crew member closed the emergency door and the plane continued on to the gate, where the passengers exited without incident, according to CBS San Francisco.