attention, memory, and booming buzzing confusion
Way back in 1891, William James wrote one of the most (over-)quoted phrases in psychology. Describing the nature of perception and development, he said: "The baby, assailed by eyes, ears, nose, skin, and entrails at once, feels it all as one great blooming, buzzing confusion." Over time, though, we learn to make sense of the world, to divide our perceptions into interesting and uninteresting, and to ignore the details of the world around us if they're not relevant to what we're doing.
Now there's some new research, mostly recently including a report published in Nature Neuroscience, that seems to show that our ability to do this task, to ignore unimportant perceptions, weakens with age. This seems to underly problems older people have with another important cognitive function -- memory. In particular, short-term ("working") memory, which refers to your ability to hold a limited number of ideas in your head at the same time. The researchers looked at how well young and older adults did at ignoring photos that they were told they should ignore, and remembering the ones they should remember. Most of the older adults were significantly less good at the task than the younger adults. Even more interestingly, the participants' brains were imaged as they performed this task, specifically a part of the brain that's believed to be involved with memory of visual events. The younger adults showed less activity when they were told to ignore the photos, and more activity (than a baseline) when they were told they should pay attention. The older adults, however, showed moderate levels of activity even when they were told they should ignore the photos! If you can't ignore things in your environment that you know to be irrelevant, then the world starts to revert, at least a little bit, to that buzzing confusion. You can't remember everything, and if you can't successfully ignore unimportant things, your ability to function will certainly be reduced.
(On the other hand, they didn't control for whether or not the older participants were just too jaded and cynical to believe them when they were told that the irrelevant photos were in fact irrelevant...!)