According to the flight crew and a flight attendant, about 15 minutes after departure, the airplane was climbing through flight level 220. Although the captain allowed the use of approved portable electronic devices (PED) as the airplane climbed above 10,000 feet, the fasten seatbelt sign had remained illuminated for the climb. When the PED use was allowed, a couple of passengers began to move about the cabin, including one that went to the lavatory. As the passenger was exiting the lavatory and returning to her seat, the airplane encountered unforecast moderate turbulence. The passenger subsequently fell down in the aisle and suffered a fractured ankle. The flight returned to the departure airport.