Grandpa Solomon, a 26-year-old rhesus monkey - equivalent to a 78-year-old human
Scientists say they have discovered why some elderly people have problems with speech, vision and mobility.
They say they happen because older people do not have enough of a certain chemical in their brain.
The chemical, called GABA, helps the brain work efficiently, but the supply appears to diminish later in life.
To take this further we need to start to look for an entirely new class of non-addictive, GABA active drugs
Dr Richard Harvey, Alzheimer's Society
Tests on monkeys have shown giving additional doses of the chemical can improve mental ability, and the researchers say their findings could lead to the same kind of improvements for humans.
US researchers have carried out studies on very old macaque monkeys, and have shown the decline could be reversed by boosting levels of GABA.
They studied the visual functions of the monkeys, who were all around 30- years old, the equivalent of 90-year-old people.
As in humans, the monkeys' eye-sight declines, partly because the vision-related section of the cerebral cortex, which is responsible for many "higher-order" brain functions degenerates. - the same degeneration is likely to take place in other parts of the cerebral cortex, say the researchers.
They say GABA helps the brain make sense out of the vast quantities of visual information which come in through the eyes.
The lack of GABA means the visual cortex cannot weed out useless visual information, clogging up the brain and slowing processing down.
The researchers recorded the activity of individual neurons in the visual cortex of old and young macaque monkeys, while showing the monkeys various images on a computer screen.
The monkeys were also given GABA, a GABA-enhancing compound called muscimol, and a GABA-blocking compound called bicuculline.
The GABA blocker made the neurons less selective in the young monkeys, but had no significant effect in old monkeys, perhaps because the older neurons had already lost much of their selectivity.
GABA and the GABA-enhancer had a slight effect in the young monkeys.
But in the old monkeys, GABA and the GABA-enhancer had a much stronger effect, significantly increasing the percentage of the visual-processing cells.
Drugs, such as Xanax, do exist which can increase GABA production.
Dr Audie Leventhal, one of the researchers on the study, said: "The good news is there are a lot of drugs around that can facilitate GABA function and maybe some of them will help."
But he said no testing had been done on the elderly.
"Hopefully we can drum up a little interest and encourage other people who are trying to figure out how come their kids are smarter than they are now."
Dr Richard Harvey of the UK's Alzheimer's Society, told BBC News Online: "Understanding the changes that occur in the brain as we age is fundamental to tackling the mental decline that adversely affects quality of life for many older people."
He said understanding how the neurotransmitter GABA worked could help scientists overcome some of the effects of ageing in the brain.
Dr Harvey added: "The challenge will be to find ways to do this without causing unwanted side effects.
"The benzodiazepines, which include Xanax (alprazolam) and Valium (diazepam) affect the GABA system in the brain.
"However they are highly addictive, and any benefits you might get from enhancing GABA are mitigated by the significant problems of physical and psychological dependence.
"To take this further we need to start to look for an entirely new class of non-addictive, GABA active drugs."
The research is published in the magazine Science.