At two I was a committed vegetarian, at four a committed socialist, at six a committed atheist. I could calculate the kinetic energy of a bicycle in motion before I could ride one. At four years old, I most certainly did feel de-humanised when refused a vegetarian meal or spitefully tricked into eating meat. I'm obviously a distant outlier, but the standard deviation is much greater than most people imagine. At the other end of the distribution, here in Britain a tenth of all 16 year olds can't understand a bus timetable, fill in a job application, read the instructions on a frozen meal or make change from a ten pound note. I believe matters are significantly worse in the US. Those people have all been dismally failed by a society and a schools system that assumes that children develop at the same pace. Given that in my country half of all prisoners are functionally illiterate, I believe we have paid a very dear price for that attitude.

Age-appropriate is one size fits all; It works fine for average people but fails catastrophically for anyone at the edge of the bell curve. If you think there is such a thing as "age appropriate learning and guidance", spend a day in an elementary classroom and see for yourself just how different a group of young children can be in terms of mental and emotional development. I'll wager that you'll be astonished at just how great the range is.