From MSNBC.COM:

Brains of very smart kids mature later

Study: Delayed development of cortex may promote higher intelligence

NEW YORK - Very smart children may seem advanced in many ways, but a new study shows they actually lag behind other kids in development of the “thinking” part of the brain.

(more here)

So that slow kid sitting next to you in 2nd grade…  Watchout, he may be a Rocket Scientist some day.