(FORTUNE Magazine) – Your servant senses that it is time for a little more back talk on the subject of ageism. Every time you turn around these days, there is another uplifting editorial deploring bias against the oldies, averring that age is really quite subjective, and noting that octogenarians compete in tennis matches. The uplifter at the Christian Science Monitor was recently citing not only the senescent tennis stars but the nonagenarian lady who had climbed Mount Fuji. In the last couple of months, Congress passed and the President signed two more irrational laws expanding the ban on age discrimination. Meanwhile, the existing irrational law -- the Age Discrimination in Employment Act of 1967 -- continues to generate an avalanche of farcical court cases in which somebody or other is suing the company, typically because age was a factor in a dismissal or nonpromotion. Luckily the Seventh Circuit Court of Appeals at least assented recently to the Federal Aviation Administration rule grounding commercial pilots at 60. Age discrimination is rational because in the typical case, critical mental functions decline with age -- and because it is ordinarily not practical to test the brainpower of employees claiming to be untypical. As you age, your skull thickens and your brain shrinks. By age 65, the brain has typically lost about 6% of its weight at age 20, and IQ scores are perhaps 15% lower. To be precise, what declines are the ''raw scores,'' i.e., before the usual age adjustment. (People taking IQ tests get scores normed against others in their own age cohort.) It is true that the decline in mental ability is not across-the-board. People tend to hold up pretty well in ''crystallized intelligence'': the ability to perform familiar tasks. The IQ subtests measuring vocabulary and general information show little slippage, for example, even for folks in their late 60s. Where people crash with age is in the test items measuring ''fluid intelligence'': the ability to learn new tasks, see things in different ways, respond creatively to unfamiliar challenges (as you hope the pilot in the airline cabin would be able to). On the Wechsler Adult Intelligence Scale, the IQ test an old-timer is most likely to take, the age-based declines are extremely severe in a subtest where you are given a scrambled group of pictures and asked to sort them into a logical sequence. Raymond Cattell, the psychologist who first elaborated the fluid- crystallized distinction a half-century ago, came to mind the other day when we were reading about the latest row over age discrimination. The row, played out in the New York Times letters column, dealt with a familiar issue: Should college professors be forced to retire at 70 (as they may be under existing law)? Writing in for the negative was Philip B. Kurland, 69, an eminence of the University of Chicago law school. Nutshell brief, his point was that, sure, older professors might lose some of their creativity and originality, but they retained their judgment -- and ''judgment is what higher education is all about.'' Cattell did not agree with that assessment. He observed that old folks are forever complaining about their increased forgetfulness but that you never hear them talk about their declining judgment. In fact, he added, judgment -- basically synonymous with reasoning ability -- declines more rapidly than memory. Possibly even more rapidly than the backhand.