Due to the age of the car, I imagine various seals and gaskets are starting to deteriorate and cause small leaks. It's possible the piston rings may also be worn and allowing oil into the combustion chamber (where it is burnt off).

At 12 years old, the car is approaching the end of its mechanical life (most cars are scrapped at 14-years). You might be ok for another two or three years by topping up the oil but there is no way to fix age-related seal/ring problems without spending huge amounts of money on the car. However, at the car's next annual service in June, I would ask the mechanic to check the engine over for any obvious signs of oil leakage.