During this time, and presumably underlying these changes, the brain continues to mature via processes such as synaptic remodelling and competitive elimination, programmed cell death and myelination [20, 21]. Structural imaging studies demonstrate a linear increase with age in white matter, presumably reflecting myelination, that peaks at around age 50, and a non-linear decrease in grey matter density and cortical thickness, presumably reflecting synaptic pruning and myelination, up to age 40 [22, 23]. These processes are heterochronous and heterogeneous with higher association areas in frontal, parietal and temporal regions maturing latest and primary sensory areas maturing earliest [22, 24]. Gender differences show that males exhibit steeper developmental slopes in grey matter reduction and white matter increase than females, partly explained by earlier maturation peaks in females in frontal, striatal, temporal and parietal areas [25, 26] Gender differences in cognitive abilities may at least in part be explained by these sex differences in brain structure and its development [25].
