A common feature of many fantasy world, both the "realistic" fantasy and the "fantasy" fantasy is that the time scale of "historical" events is way off as compared with our own.
For the most famous example, in Tolkien's world more than 7000 years passed between the time men appeared, already possessing some basic form of civilization (and quickly provided with more by the elves), and the end of the Lord of the Rings. In this 7000 years the technology, politics, culture, religion, and whatnot, remained virtually the same (or actually degraded to a lower level over some periods), at a level that stands roughly somewhere between our World's antiquity and middle-ages (or early modern in some settings, such as the Shire). By comparison, in real history, it got only a little more than 5000 years to go from the onset of the Egyptians people (used by Tolkien as a comparison with Gondor, although most of the people of first, second and third era seems definetely to be more advanced than ancient Egypt) to space shuttles and supercomputers. More on this here (do not click unless you are prepared to yield control over your life for the next 6-12 hours).
Since it is really too cool to say things like "ten thousand years ago... etc etc", I was thinking if there could be a simple way to rationalise this in low-fantasy settings, that is without immediately recurring to "it's magic".
And here is the idea I would like to have commented by you: the technological explosion in our world depended on many factors, but one that stand out is the use of fossil fuels. True, we had the scientific revolution, or at least the begin of it, before that, but technology really started to change everyone life only after the abundant reserve of energy stored in fossil fuels became to be harnessed.
So, if a world has no fossil fuel, or no fossil fuel in a significant quantity, it probably couldn't develop in the same way and would be stuck, at most, to an early-modern level, probably with a cycle of progress and regress as civilizations rise and fall.
Now, the easiest way to explain the lack of fossil fuels, it seems to me, is the lack of time for it to form, that is to say that there are not the hundreds of millions of years between the emergence of plant life and the "current" historical moment in the fantasy world. It is perhaps possible, or at least imaginable (maybe here some little magic, or any supernatural thing, could help to explain), that evolution took a different route, and animal species evolved up to human (or whatever is the dominant species in said fantasy world) in the same time-frame it took for vegetable life to evolve at the same level we have now (which, after all, has been relatively stable for a helluva lot of time, hasn't it?), without enough time to produce fossil fuels in significant quantity.
Therefore, a world with a long long historical time would be a world with a short short geological history. This would very neatly cohere with another trope typical of fantasy, that of the primevally rough landscape, "when the world was young", strange primitive beasts, etc etc. Plus, and this is truly the start of all my train of thoughts, would explain why my world appears geologically so young – which in reality is due to the fact that I have been unable to apply proper erosions patterns (that is, rivers are ok, but wind is nonexistent, so that true plains are scarce, and mountains are all very sharp) to the world I created starting with Fractal Terrains!
What do you think?
And yes, I know that I am crazy, in case you were asking yourself.