Experiments on mice suggested low levels of a protein in the brain may be responsible for memory loss.
It is hoped the discovery could lead to treatments to reverse forgetfulness, but it is a big leap from the mouse to a human brain.
The study, published in the journal Science Translational Medicine, said age-related memory loss was a separate condition to Alzheimer’s disease.
The team at Columbia University Medical Centre started by analysing the brains of eight dead people, aged between 22 and 88, who had donated their organ for medical research.
They found 17 genes whose activity level differed with age. One contained instructions for making a protein called RbAp48, which became less active with time.