As has been well documented, Working memory (WM) capacity changes with age, increasing in childhood and undergoing a relatively early decline in older adulthood, just like fluid intelligence. Several mechanisms have been hypothesized to account for this decrease in the functional capacity of WM. One of these processes, empirically well documented, is processing speed. Salthouse, for instance, has shown in many studies that a decrease in processing speed accounts for a large part of the age-related variance in WM tasks. A second mechanism has more recently been incriminated, after the seminal work of Hasher & Zacks, namely inhibition (or resistance to interference). If the inhibitory processes are not very efficient (as in young children or in older adults), WM may be encumbered by irrelevant information, that is, either by really irrelevant information, or by information that is no longer relevant, and performances will be lower. However, most often, researchers have considered processing speed and inhibition as alternative potential mediators of age effects on WM, whereas they could be considered to be complementary. Also, even though similar hypotheses have been advanced with respect to the increase of WM during childhood, they have been subjected to less empirical scrutiny.

After a brief overview of these various hypotheses, a research project will be presented, the objective of which was to study the joint influence of inhibition and processing speed on working memory in childhood (8, 10, and 12 year-olds), young and older adulthood. The presentation will focus on the empirical results of this study, centering on age differences in each of the tasks used and on the amount of age-related variance in working memory performance which was accounted for by inhibition and processing speed, using multivariate analyses (regression and commonality analyses and structural equations modeling). It will be shown that processing speed did indeed account for most of the age-related variance when comparing either children or older adults with young adults; however, the relative weight of inhibition appeared to differ somewhat in older adulthood and in childhood.

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