Adam Gazzaley of the University of California ran tests where he asked two groups of people, one aged 19 to 33 years and the other group aged 60 to 72, to perform memory tasks while wired up to an electrocephalogram machine.
What they found was that the speed of the brains in older and younger people was similar, but that the older participants performed less well on the memory tests because their brains were not as good at blocking irrelevant information. They experienced more distractions in the early part of each test and it was this delay that caused them to perform less well.
However, when the older participants were divided into the better performers and those who did less well, the results from the better performers were comparable with those of the younger people. This slowing of the brain is therefore not something that happens to everyone, but the tendency for it to happen does increase with age.
The next step in the research is to find out why it happens and what can be done about it.
On the other hand, I think I quite like my ability to be distracted!