Well, let me tell you something. After 25 years in the classroom at all levels from grades 1 through 12 and beyond, IT JUST AIN'T SO. I dearly love teaching first graders art. They are the sweetest, most endearing little creatures in the world, but I see just as many "uncreative" first graders as I do "uncreative" twelfth graders. The ratio doesn't change appreciably as the child grows older. The educational system, at least as it's now constituted, does NOT stifle creativity. I've come to realize that creativity is NOT a fragile entity. I suppose it might be possible, under exceptionally constrictive circumstances to so brow-beat a child that all creative juices are squeezed out, but American schools today are anything BUT constrictive.
I think how this "old wives tale" regarding the degeneration of creativity developed due to the fact that, from first grade artists, parents are willing to accept just about any childish scrawl as a "masterpiece" without regard to whether the child has the skill to exploit his or her creativity. As the child matures, they discover that to express their similarly maturing ideas they need similarly more and more mature skills, which may or may not be at their disposal. As an educator, I am not blinded by any all-embracing love for each and every incomprehensible first-grade scribble of crayon on paper. I've found first graders are just as capable of trite, stale, inhibited, unoriginal, drawings as the senior in high school forced by the system to take art to satisfy a fine-arts credit needed for graduation.