depends upon efficiency. do you get your oil changed regularly? got everything 'lubed up?
as a rule of thumb, when cars get older their mgp decreases because friction has taken its tole (wear). that said, keeping your car in good condition can slow this wear to a minimum.
but you only have 39k miles...you really shouldn't be noticing THAT much. IMO, i'd say it was probably oil changes, but it doesnt have to be that.