A new study has found that dementia rates among people 65 and older in England and Wales have plummeted by 25 percent over the past two decades, to 6.2 percent from 8.3 percent, a trend that researchers say is probably occurring across developed countries and that could have major social and economic implications for families and societies.

Another recent study, conducted in Denmark, found that people in their 90s who were given a standard test of mental ability in 2010 scored substantially better than people who had reached their 90s a decade earlier. Nearly one-quarter of those assessed in 2010 scored at the highest level, a rate twice that of those tested in 1998. The percentage of subjects severely impaired fell to 17 percent from 22 percent.

The British study, published on Tuesday in The Lancet, and the Danish one, which was released last week, also in The Lancet, soften alarms sounded by advocacy groups and some public health officials who have forecast a rapid rise in the number of people with dementia, as well as in the costs of caring for them. The projections assumed the odds of getting dementia would be unchanged.

Yet experts on aging said the studies also confirmed something they had suspected but had had difficulty proving: that dementia rates would fall and mental acuity improve as the population grew healthier and better educated. The incidence of dementia is lower among those better educated, as well as among those who control their blood pressure and cholesterol, possibly because some dementia is caused by ministrokes and other vascular damage. So as populations controlled cardiovascular risk factors better and had more years of schooling, it made sense that the risk of dementia might decrease. A half-dozen previous studies had hinted that the rate was falling, but they had flaws that led some to doubt the conclusions.