Here's a nice research review of selective attention in Trends in Cognitive Neuroscience. Here's one interesting finding:
Older adults and younger school age children had problems with focus at low levels of 'visual challenge' (the task was to hunt for a letter in the setting of side distracting letter). When the task was made more challenging (centrally distracting letters), both the older and younger subjects performed better!
Think about this. This means that a significant component of visual distractibility is under cognitive control. For the school age child or older adult, hard is easy, and easy is hard. Maybe this should not be as counterintuitive as thought though. When are you most likely to doze off or daydream in a lecture - when you know most of the material and it all looks too elementary? or when you're presented with an intellectual challenge?
It also suggests that it might be wise to not be dismissive of age-related attention-focusing problems. There may be a lot to work with...
Perceptual Load and Attention