As a technology platform, the manipulation of stem cells is very broad indeed. Once the fundamental methodologies and infrastructural technologies involved in sourcing and altering stem cells are made reliable and cheap - a process that is still very much underway - then the sky will be the limit. Any part of the human body will be up for at least some benefit and quite possible outright replacement: mature stem cell medicine will provide the closest thing yet to a general repair kit for human beings.
Much like a general repair kit for cars, stem cell medicine cannot be used to evade the end results of aging - you can slow down the consequences, but there are aspects of aging that cannot be tackled using these tools. Cell replacement and manipulation is only one of a number of different technology platforms required to restore age-damaged biochemistry to a youthful state. But it is a step in the right direction.
Here are a couple of articles on recent work at opposite ends of the stem cell research field - reversing degenerative blindness and muscle loss. They are representative of stem cell science at present: a fast pace of discovery, new technology demonstrations every week, and a lot of promise when it comes to near-future applications in medicine.
Using stem cells salvaged from the retinas of human cadavers, researchers with the University of Toronto have restored sight to the eyes of, well, three blind mice. The feat, aside from indicating a quirky sense of humour, has been repeated several times over the last year and marks an important step toward the goal of restoring sight in people. ... All the basic biology of the human eye and retina is the same as it is in the mouse. If we can get enough of the cells to grow and integrate, I think we’d go right into [trials with] humans.
A happy accident may hold the key to healing muscle diseases and granting humans incredible physiques. Researchers at the University of Colorado at Boulder and the University of Washington discovered that stem cells injected into mouse muscles led to increased growth for the rest of the mouse’s life. Young mice with injured legs were given donor muscle stem cells from other young mice. Those injuries not only healed, but muscle mass increased 50% and muscle volume increased by an incredible 170%! Performance tests show the muscles were twice as strong as normal, and still above average when you control for size. Two years later, about the lifetime of a mouse, the legs were still bigger and stronger than normal, much to the scientists surprise.
The paper for that second article is entitled "Prevention of Muscle Aging by Myofiber-Associated Satellite Cell Transplantation". You might look back into the Fight Aging! archives for more on satellite cells in muscle and aging - decline of function and/or numbers in this stem cell population has been a hot topic in the research community for a few years now: