Scientists at the US National Institute for Mental Health have discovered that a chemical in the brain governing the delivery and feeling of reward is altered physically as a person grows old.
The findings help to explain why it is that children find it almost impossible to contain their excitement on special occasions such as birthdays and Christmas, whereas the older we become the less excitable we tend to be.
"Knowing how key brain circuits change as we get older may help us to rise to the public health challenge of ageing successfully," the Independent quoted Karen Berman, as saying.
The study has been published in the journal Proceedings of the National Academy of Sciences.
The results could also lead to new treatments for conditions caused by defects in the brain's "reward system", such as drug addiction and Parkinson's disease.