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.
Also, experience
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).
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.