Does the speed of electrical impulses through neurones decrease with age?
As @Armatus notes, it is now generally understood that age-related cognitive decline is related to loss of myelination and axon damage (see, for example, Kohama et al, 2012). Myelin degeneration is also likely the cause of some age-related peripheral neuropathies. Loss of myelination (due to inflammation) is also the mechanism behind multiple sclerosis.

Myelin allows for fast and reliable transmission of action potentials by a mechanism known as saltatory conduction. This is necessary to compensate for axons which have small diameters (unlike the squid giant axon) and travel long distances. Degraded myelin will reduce conduction velocity and can result in propogation failures. The links @nico provided in his comment above are useful references.



Kohama, S.G., Rosene, D.L., Sherman, L.S., 2012. Age-related changes in human and non-human primate white matter: from myelination disturbances to cognitive decline. Age (Dordr) 34, 1093–1110.