I wouldn't worry too much with age of an engine. It can be a bit "tired" (usually near TBO when the owner doesn't want to "waste" the engine resource by overhauling the engine sooner than TBO, even if there is proof that the engine should require overhaul), but usualy the problem is that you achieve far less than rated power - the fuel consumtion should stay more or less the same. On the other hand, the most common problem with people having higher fuel consumption / less range than the POH figures is because they don't read the "small print" on the performance charts.
For example, the POH for Warrior III suggests a fuel flow of 9,2 USGal/h for 65% power. Right next to the fuel flow it says "Best Economy Mixture" - this is the important part! You won't get this figures by leaning to max RPM, and than moving a mixture half an inch forward for every flight instructor that said "don't touch the red knob" to you. If you then look at the Cruise section of Normal Procedures, you will see that the required procedure to obtain the noted fuel flow for best economy is with full throttle, and mixture leaned to achieve RPM stated in the table. Otherwise very, very strange method, especially with carburetted engines at low altitudes.
Basically, operate the aircraft as the book says, note IAS/TAS which you achieve and compare it with the burned fuel and you'll be able to see where you stand.