No idea about Boeing, but Airbus has a schedule where you can calculate the theoretical deterioration in fuel burn over time. It is then recommended that you apply this extra fuel burn to your flight planning. We have A320 MSN805 which has +7.8%! I believe that you can reduce this figure if you monitor the actual burn performance.
As to why the burn increases, if the engines are the same type (and replaced every few years), it can only be weight or airframe. I can only guess that the airframe gradually bends slightly out of shape.