"The driver of the car left the scene without stopping," Supt. Mike McGrath of the Lower Merion Police said.
Police are looking for a light colored station wagon, possibly a gold or tan Volvo, with damage to the passenger side of the front windshield.
The driver is said to be a white woman, 30 to 40 years old. She was last seen driving north on Morris Road toward Spring Mill.
The victim, a 13-year-old boy, was airlifted to CHOP. He is said to have head trauma and injuries to his lower extremities and a possible broken leg. Police say he was not was wearing a helmet.
So far, police have not been able to find a witness who saw the tag number, but they believe someone out there may have more information because the area is well traveled.
"This is the area around Bryn Mawr College. It's a mix of institutions around there, you have Baldwin School, Bryn Mawr College, Shipley School, and residential all mixed in within that neighborhood," McGrath said.
Anyone with information is asked to call Lower Merion Police at 610-649-1000.