Stem cell therapies offer all sorts of possible ways to intervene in disorders of the brain and nervous system: the evidence suggests that, as for other parts of the body, there is a lot that can be achieved by dropping in a bunch of fully functional cells of the right type and letting them get to work. Or, alternately, by finding ways to stimulate existing cell populations into working harder.
Most of these approaches fall into the category of patches: increasing the pace of repair and recreation of destroyed resources, but doing little to address the underlying reasons for damage and destruction. As a strategy this is second-rate, especially in the brain, but it is how the mainstream of medical research proceeds. In part we can blame regulatory bodies for the focus on patching end results rather than preventing root causes: the way in which requirements and costs are imposed on the development of new therapies leads to a situation in which it the less expensive (and in some cases only) path is to build treatments for late stage disease.
Damage to myelin, the sheathing for axons in nerve cells, is at the root of a number of serious medical conditions. As is the case for most of our biology the integrity of myelin declines with age; some fraction of the age-related decline in cognitive function that occurs for everyone is thought to stem from progressively less effective myelination in the brain. A number of research groups are engaged in ongoing work with stem cells aimed at the repair of myelin, and here is one example:
Neural stem cells transplanted into the brains of people with Pelizaeus-Merzbacher disease (PMD) can differentiate and begin producing the myelin sheaths that these patients lack, according to results of a Phase I clinical trial. ... If the stem cell transplants do ultimately demonstrate benefit, they could help more than just PMD patients ... There's a wide range of possible myelin disorders that could be targeted, including demyelinating disorders like multiple sclerosis and preterm babies at risk for cerebral palsy due to white matter injury.
Here, as in many other cases, a therapy is under development for use with specific named diseases - but it might also prove helpful as a treatment for aging, as an attempt to retard loss of cognitive function. Yet there is no path to legally produce therapies for general use in all old people in the US: the FDA doesn't recognize treatment of aging as a legitimate use of medicine, and short a revolution there's little hope of changing that situation through existing paths. Until this changes, a great deal of promising work will be sidetracked into narrow usage for late stage specific diseases, and any real progress towards clinical applications for aging will have to happen outside the US research community.