The aims of this research were to investigate the nature and onset of cognitive decline. The thesis discussed limitations of the current design and investigated domains of cognition likely to have diagnostic utility for early detection of cognitive decline. From the results obtained, a modular model is endorsed with cognitive reserve masking decline in certain circumstances. Consistent with recent research, there is evidence of decline as early as the second decade of life. This thesis has also outlined a potential pathway for the development of further research into the area of aging.