Mental acuity is very much a physical quality. Yes, there are 70 year olds who can think and mentally adapt exceptionally well, just as there are 70 year olds who can do one arm push-ups and live till 100. But it doesn't mean that 70 year old are just like 30 year olds, but with more experience.
Most of the studies that showed rapid mental decline in old age have been debunked. A 70 year old with no actual problems (e.g., Alzheimer's) is very likely to function perfectly fine mentally.
A 2002 study (published in the Alzheimer's and Dementia Journal) found that ~91% of people 70 and older were capable of normal mental function.
Seriously? How many 70 year old people do you know? Let say you need to urgently take your child to a doctor and can choose between a 30 yo and a 70 yo. What would you do?
Parent cited an experiment that found 70 year-olds to have mental functioning on par with young people. Would you like to argue against the study's conclusion in a meaningful way? Or are you just pointing out that regardless of what is true, humans tend to act in a biased and ineffective manner?
That's a straw man right there. Why would it matter? In terms of the decision making, the one with more knowledge is better. In terms of driving, it depends on if the 70 year old still has the PHYSICAL ability and acuity to drive a car, which is exactly what we weren't talking about.
If you look at a site/product like Lumosity.com, you can compare your results to other age groups. If you accept that the tests on Lumosity do test particular brain functions/areas then its clear to see that from the millions of user that take part, the performance average does decline with age. It's actually quite dramatic to see that a 40 year old that scores in the 90% in a test category, fits in at 50% among the 20 year olds.
You can't really get any meaningful results from a self selected sample.
Since there is no other evidence that a 40 year old in the 90th percentile for mental performance is equal to a 20 year old in the 50th percentile, it seems more than likely that the data is skewed.
Perhaps Lumosity.com attracts above average young people and below average older people (given those results, some variation on that theme is the likely explanation).
An otherwise almost imperceptible decline in mental function could have have a huge impact on ranking among top chess players simply because at the top levels the margins between players are so small.
If chess Grandmasters dropped in mental acuity as fast as those Luminosity results suggested, a 60 year old Grandmaster could be beaten by any young novice.
Also worth mentioning is that competitive chess is favors very quick thinking and action in a way that most careers don't.
