Here is an interesting article about the increasing difficulty of assessing dementia in the eldery, compared to thirty years ago. The article discusses research that suggests older adults today retain far more of their cognitive skill set that used to deteriorate with the onset of dementia, with only the loss of memory remaining a significantly impacted function. From the article:
Several of the tests previously used to predict which elderly individuals risk developing dementia do not seem to work any longer. The thesis shows that memory loss is the only factor that can still be used to indicate who is at risk, although not among the very old.
The study compared nondemented 70-year-olds examined in the early 1970s with nondemented 70-year-olds examined in the year 2000. The results show that those who were examined in 2000 scored much higher on psychological tests than those examined 30 years earlier. This finding clearly indicates that such tests can no longer be used to predict future dementia. In the early 1970s, several different tests could be used to predict people's risks of developing dementia, but today it seems like psychiatric evaluation of the memory is the only useful test. In addition, it is more difficult to predict dementia the higher the person's level of education, says physician PhD Simona Sacuiu, the author of the thesis.
Unfortunately, the article does not go into details regarding the tests utilized, which would allow for a better understanding of what the researchers saw. Overall, though, with an increasingly educated population that has seen the demand for cognitive effort increase as well (as Steven Johnson has noted), it makes intuitive sense that slippage caused by dementia would be harder to detect in the early stages. On the other side, it will be interesting to see if brain inaging will, in the long run, help out in terms of early detection of dementia - hopefully coinciding with an increase in effective interventions.