MS, which affects more than 10,000 people in Scotland, occurs when the immune system destroys myelin, the fatty insulating layer that protects nerve fibres in the brain and spinal cord.
Symptoms can range from mild numbness and tingling to vision loss and crippling paralysis.
Early in the disease, the myelin can repair itself to some extent and maintain normal nerve function.
However, as the patient ages, this ability – known as remyelination – is increasingly lost, making treatment much more difficult. Less myelin is restored until nerve fibres are permanently destroyed.
Scotland has the highest rate of MS in the world, with a combination of genetics and environmental factors, including low levels of vitamin D, thought to cause the condition.
The new study on mice shows that the age-associated decline of remyelination can be reversed.
When old mice were exposed to immune cells taken from the blood of young mice, the myelin which was covering their spinal cord nerve fibres began to regenerate.
The discovery could pave the way for new therapies for MS, according to the British and US scientists whose work is reported online in the journal Cell Stem Cell.
Professor Robin Franklin, director of the MS Society's Cambridge Centre for Myelin Repair at Cambridge University, said: "What we have shown in our study ... is that the age- associated decline in remyelination is reversible.
"We found that remyelination in old adult mice can be made to work as efficiently as it does in young adult mice.
"For individuals with MS, this means that in theory regenerative therapies will work throughout the duration of the disease.
"Specifically, it means that remyelination therapies do not need to be based on stem cell transplantation since the stem cells already present in the brain and spinal cord can be made to regenerate myelin – regardless of the patient's age."
MS affects around 100,000 people in the UK, 400,000 in the US and several million worldwide.
In the study, small patches of spinal cord myelin were artificially destroyed in old mice, mimicking the effects of MS.
Those areas were then exposed to "macrophage" immune cells taken from the blood of young mice.
The immune cells helped resident stem cells to manufacture new myelin and repair the damage.