Although it’s too soon to sound the death knell for the “senior moment,” it appears that memory loss and thinking problems are becoming less common among older Americans. A new nationally representative study shows a downward trend in the rate of “cognitive impairment” — the umbrella term for everything from significant memory loss to dementia and Alzheimer’s disease — among people aged 70 and older. The prevalence of cognitive impairment in this age group went down by 3.5 percentage points between 1993 and 2002 — from 12.2 percent to 8.7 percent, representing a difference of hundreds of thousands of people.