On becoming a "third-grader" –  Steve Catton

One way to think about the phenomenon "third-grader" is to visualize him or her as being just about halfway on a number line with its two endpoints birth and adulthood.  With the units labelled years, there, right in the middle -- at least by the end of third grade or beginning of the fourth -- would be the nine-year-old who has reached or finished third grade.

If you set the endpoints as length-at-birth and height-at-adulthood, again you'd find your "typical" third-grader approaching the middle.  It gets a little more subjective beyond that, but I reckon they're generally approaching the halfway point on the road to social and intellectual maturity, too. Halfway is a pretty noteworthy milestone.  And it's a pretty special point from which to view the way forward and backward.