White matter appears to decrease faster than grey matter, but doesn't begin to decline until the forties. Presumably this relates to the decline in processing speed that is the most evident characteristic of age-related decline.
Grey matter, on the other hand, declines at a fairly constant rate from adolescence, mirroring a decline in processing ability that seems to start as early as the twenties.
Cognitive decline seems to be faster in women than men. This presumably reflects apparent gender differences in brain activity. For example, while women seem to have a greater density of brain cells in the prefrontal cortex, they also show a steeper rate of decline so that, in old age, the density is similar between the genders.
There is some evidence that individual differences in processing speed and memory are more important than age, and that personality attributes affect the rate of cognitive decline and brain atrophy.
Some gene variants, including the so-called Alzheimer’s gene, are associated with a faster rate of decline, or an earlier start. These may be triggered by activity in early adulthood. Head size in adulthood also seems to affect rate of decline. Head size in adulthood reflects not only head size at birth, but growth in the early years — pointing to the importance of providing both proper nourishment and intellectual stimulation in these early years.