Today's 90-year-olds are surviving into very old age with better mental performance than ever before, Danish research suggests.
People born in 1915 scored higher in cognitive tests in their 90s compared with those born a decade earlier, according to a study in The Lancet.
Better living standards and intellectual stimulation may be key factors, experts say.
The number of people reaching very old age is on the rise globally.
In the US, for example, the number of people aged 90 or above has more than doubled in 30 years.
In Denmark, where the study took place, the chance of surviving into the 10th decade of life has gone up by about 30% each decade for people born in 1895, 1905 and 1915.
However, there has been little research on the quality of life that people reaching such an old age can look forward to.
The researchers, led by Prof Kaare Christensen, of the University of Southern Denmark in Odense, surveyed all Danes born in 1905 who were still alive and living in the country in 1998 (3,600 people, aged 92-93).
They assessed their physical strength, mental functioning, ability to carry out daily living tasks such as walking inside and outside, and any symptoms of depression.
Twelve years later, they repeated the study with Danes born in 1915 (2,509 people, aged 94-95).
The researchers found that men and women born in 1915 performed better than those born in 1905 in terms of cognitive ability and activities of daily living, even after correcting for changes like better education.
Prof Christensen and colleagues said: "Our results show that the Danish cohort born in 1915 had better survival and scored significantly better on both the cognitive tests and the activities of daily living scale than the cohort born in 1905, despite being two years older at the time of assessment.
"This finding suggests that more people are living to older ages with better overall functioning."
The research addresses the key question of whether living into very old age is accompanied by more years of poor health, or whether overall health at an advanced age is improving.
Commenting on the study, Prof Tom Kirkwood, associate dean for ageing at Newcastle University, said the data from Denmark was "encouraging".
"It seems that among those born in 1915, cognitive function in advanced old age is measurably better than for those born in 1905, even when underlying changes like improved education are taken into account," he said.
In the UK, the most complete picture of health in advanced old age comes from the Newcastle 85+ study, which has been looking at people born in 1921.
The investigations will be repeated in those born a decade later, giving the chance to see if the Danish findings apply in other populations.