Thanks. That there is signal in which twin is taller at precisely age 12 is interesting, and seems like the same phenomenon as the adult effects.

But I'm saying something much simpler. In a school class, there's another effect, that the oldest may be a year or more older than the youngest. The mix of ages in a classroom will produce a correlation all by itself. It seems to me this would be a much bigger effect: 7th-graders are quite a bit taller, and better on tests, than 6th-graders.