In favor of labeling this decreased GFR in the elderly as abnormal, histopathologic analysis of geriatric kidneys shows evidence of microvascular disease that in a young person would not be seen as normal. Mortality does seem to correlate with GFR, even in the mid-range. Also, from a historical perspective, the idea that hypertension was driving conditions such as stroke and cardiovascular disease was initially dismissed based on how common it was in the general population; the same situation could conceivably be the case here. A good review, from a 2005 NEJM article, is found here.
In opposition to this, Drs. Glassock and Winearls rebuttal in this month’s AJKD take issue with the use of the pejorative term “disease” to describe such a large fraction of the nation’s elderly, and suggest instead to use the term “age-related reduced kidney function.”