Gray matter diminishes as the chld develops because, as the brain matures, neural connections are "pruned". (This is NOT a loss of brain matter or brain function: neural structures which embody little used early learning are sacrificed so networks which embody more important learning will become stronger and more consolidated.)
~ red = more gray matter
~ blue = less gray matter
~ areas perrforming more basic functions mature earlier
~ areas for higher-order functions (emotion, self-control) mature later.
~ pre-frontal cortex is among the last to mature. It handles reasoning and other "executive" functions which emerged late in human evolution,
The brain changes in schizophrenia may be an exaggeration of this developmental pattern.
This sequence of brain changes is reversed in Alzheimer's disease.