I can hear the response already: “But how can that be? The Original Mane Six are independent, responsible individuals starting adult lives; the Human Mane Six are high-school students!”
Indeed so, but the paradox is purely illusory. To understand why, we must go back to two critical facts stated by Lauren Faust and to real-world history.
Ms. Faust has commented more than once she imagined the Mane Six as being (the equivalent of) twelve to
Modern is the key word here: Ms. Faust also has stated Equestria as she envisioned it did not have electricity. In the real world, electrical grids started developing in the 1880s, which establishes a pretty clear cap on the time period she had in mind. Steam locomotives and electrical telegraphy entered commercial service in the 1820s to 1830s, though the way they are depicted in Equestria argues for the 1860s at the earliest.
This period, the 1860s through 1880s, is smack in the middle of the Industrial Revolution. Everything was changing, especially education. The traditional apprenticeship system was strained far beyond its limits, and eventually collapsed. It was replaced by the German-originated formal, and relatively lengthy, schooling we see today, designed to mass-produce the (comparatively) well-educated work force an industrialized society requires. That transition took place throughout the nineteenth and early twentieth centuries.
Before then, and even well into that period, teens were seen as junior adults, with all the responsibility and latitude that implies. The Original Mane Six are just such junior adults, setting out on their lifelong journeys together.
As society settled into the new pattern, teens became regarded as senior children, thanks to the enforced extension of economic dependence while they spend the years needed to absorb huge stores of knowledge. The Human Mane Six are in that position.
If all this seems strange and awkward, it is. When one looks closely, the social strains imposed on teens by the conflict between biological and economic realities are clearly visible—but that is a topic for another essay.
Hints of Ms. Faust’s original intent still can be seen in both series, despite the abundance of anachronisms in Friendship Is Magic. Thinking on it now, I wonder if the collision of her determination to stay true to her nineteenth-century vision with Hasbro’s desire to present a world more accessible to a young contemporary audience was a factor in her departure from the show. I also wonder if Mark Twain’s Tom Sawyer and Huckleberry Finn were among her literary inspirations.