Cortical thickness varies by more that one millimeter between brain regions and between individuals. Genetic variation may account for some of the individual differences.
Man has approximately 30 000 genes, but there is a large number of possibilities for individual variation including variation in single nucleotides. Variation in the APOE gene (golden region in the figure above), is associated with risk of developing Alzheimers disease and affects neuronal health and plasticity through the life span.
Mental speed declines and reaction time increases with age, but individual differences also increase, meaning that trajectories of age-related decline vary markedly between individuals. The influence of genetic variation on cognition may increase with increasing age.