could be the gas station you're filling up at has contaminated tanks.... i've had that happen and seen what its done to customers cars.... lots of valve train failure issues...... i would try filling up at a different station and see if your mileage goes up.... you could always try resetting your fuel trim if it is the computer.... disconnect your battery for a few minutes and recconect it.... if you had a nice scanner your could watch the numbers as your fuel trim recalculates.... age doesn't neccisarily have anything to do with it though... my 89 nissan has 160k miles on it and still runs strong.... just about needs a new clutch now but thats fine since its the original...