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 likely 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 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 severely impaired fell to 17 percent from 22 percent.

Scans of Alzheimer's sufferer brain.

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 steadily rising population of baby boomers with the same odds of getting dementia as older people now, as well as exploding costs to care for them.

And experts on ageing say the studies confirmed something they long suspected but lacked good evidence to prove: Dementia rates would fall and mental acuity improve as the population grew healthier and better educated. The studies' findings may also give new impetus to efforts to get people to quit smoking and take other steps to lower their risks of heart disease and stroke.