They say their work, reported in Science Translational Medicine, may have promise in treating muscle-wasting conditions such as muscular dystrophy.
A UK expert said producing a human treatment might be difficult.
Stem cells are cells found in the body which can divide and become a variety of different types of tissue.
Scientists believe they could potentially help treat a large number of problems by helping to re-populate areas of tissue damaged by disease or injury.
A common problem in older people is muscle weakness, linked to a loss of muscle mass in the arms and legs.
This can lead to a swift fall in the quality of life for older people and in some cases increase the need for extra care and support.
The reasons for the decline in muscle cell production later in life are not fully understood, but the Colorado research is testing the theory that muscle stem cells could help arrest or even reverse this.
They took young mice, created an “injury” in their limb muscles, then injected muscle stem cells from another mouse.
Not only did the injury heal quickly, but the size of the muscle increased by an average of 170% – with a 50% increase in mass.
The surprising thing was that these gains did not evaporate over the next few months, as predicted by the researchers.
As the mice approached two years old, their equivalent of human old age, the size of the muscles remained constant.
Professor Bradley Olwin, who led the research, said: “This was a very exciting and unexpected result.
“The hallmarks we see with the ageing of muscles just weren’t occurring – the transplanted material seemed to kick the stem cells to a high gear for self-renewal, essentially taking over the production of muscle cells.”
The “injury” created in the limb of the mouse appeared to be significant in this process – when cells were injected into uninjured muscle, there was no growth.
Professor Olwin said that while the cells for these experiments were sourced from other mice, it might one day be possible to find a drug which could trigger a similar response from the patient’s own stem cells.
He said this would open the door to treatment not just for old-age muscle loss, but also for diseases such as muscular dystrophy, in which irreversible muscle wasting starts early in life.
Dr Hans Degens, from the Institute for Human Movement and Health at Manchester Metropolitan University, said the research appeared to be “exciting”, but that there were a number of obstacles which would need to be resolved before it could be considered in humans, including the need to control immune rejection when transplanting cells from a donor.
He said: “One of the worrying things for humans is the need for an injury to be simulated prior to treatment.
“In muscle wasting you would have to decide which muscles to treat, as the treatment would only affect a single muscle.
“It should also be noted that mouse muscles are considerably smaller than human muscles – you may have to make multiple injections.”