I was wondering if an engine would tend to run hotter as it ages? Maybe due to leaner settings to compensate for gradual decrease in compression in an effort to produce power? Fact or fiction...what do you think?

I can see where that make sense. Or, how about a reduce in compression causing a reduction in friction and heat?

In my engines the only time I have noticed an increase in temp is when the engine is nearing the end of its life. By this I mean that over its life the temps don't change but as it approaches the end of its life (say the last 10%) the temps rise a bit. This is also accompanied by other symptoms such as hard starting, poor idling etc etc. Its not a gradual change to answer your question.

As Grant Ed already stated it's not a gradual change. Engines usually run the fastest right before they essentially, "let go." Up until that point they will be very consistent and the only tuning that will be needed is to compensate for changes in weather (temp, humidity, pressure...). When the critical turning point happens your engine will become very erratic to tune, throttle response diminishes, the engine may seem like it's running rich and actually be very hot... and the list goes on. I have seen in my engines in as little as a couple tanks an engine go from running perfectly, to one that won't run for more than 2 minutes without flaming out and generally running horribly.