This month's Scientific American (March 2008) has an article called, "White Matter", by R. Douglas Fields about myelination in the brain and how that process affects development. As you know, myelin is white matter - the sheathing around neural axons that protects signals and speeds them up. Turns out that myelin has other properties/functions, too, of which I will just briefly describe.
In order to be effective, myelin must have exactly the right amount of sheathing around its neuron's axis. Too much or too little and the signal's timing gets messed up. Pretty critical! It turns out that neurons have a constant ratio (.6) for the thickness of an axon to the thickness of myelin sheathing wrapped around it. Somehow the developing brain cells are able to place the myelin down to the exact prescribed thickness. Except, of course, when they get it wrong, which they do with various developmental disorders, cerebral palsy, ADHD, language disorders, autism, bi-polar disease, schizophrenia and so on, including pathological lying(!).
The brain puts the myelin in place slowly over the years starting in the back (shirt collar) at an early age to the front (temples) by age 28-30. The brain forms and prunes itself until the sheathing is done, and then things are pretty well set - barring the phenomena of plasticity, which is a much subtler process. By the time a child turns 4, the myelin has been fully wrapped around the neurons in the visual cortex (so that is the optimal time to have vision therapy complete - but we rarely note problems by then!)
The author explains that the lack of sheathing on the pre-frontal cortex - executive function - is what prevents teenagers from making mature decisions -- that area is still developing and the lack of myelin means that the timing in the area is just not working yet.
The implication for practice with children is that there are critical times for completing therapies. Just as a person learning a second language after age 11 will retain an accent whereas a 10 year old child will not, there are developmental (and other) issues that can be repaired, adapted, etc. if caught before the final myelination takes place. Intensive practice in certain ways (Fields uses learning a musical instrument as a child for the example) will increase myelin thickness (probably because the axon grows wider, too), and this may allow for an impacted area to correct itself.
We can see why early intervention is critical. The author did not identify the critical age for various regions in the brain other than those given above. We will be waiting with baited breath for more particulars!
Fields also discussed the role of insufficient myelination in the pre-frontal cortex (during the teenage years) on the onset of bi-polar disease and schizophrenia. He described the failed process and the role of mutated genes.
Good article - can't wait for greater detail.