Mitochondria are important in aging. Each cell has its self-replicating herd of mitochondria, which play a vital role in many processes. Unfortunately they become damaged over time. Most damage to individual mitochondria can be rescued since they have limited repair processes, are recycled by other cellular processes when damage is detected, and can divide and merge like bacteria. Further they are more like enclosed bags of liquid than fixed machines, and promiscuously swap component parts with with another inside a cell. Cells can even deliver whole mitochondria to one another when in close contact.
Thus mitochondrial dynamics are pretty complex, all told, and researchers continue to discover new aspects even now. Some forms of mitochondrial damage are persistent, however: they interfere with the process of culling damaged mitochondria, and so replicate throughout a cell making it dysfunctional and harmful to the surrounding tissue. This is one of the causes of aging.
There are a number of proposed ways to treat and reverse this problem, most quite close to realization, and some of which are more robust than others. Delivery of new unbroken mitochondria or mitochondrial parts will bring short-term benefits but it won't last - the damaged forms of mitochondria will win out again, just as they did in the first place. Nonetheless, more researchers are looking into this sort of approach than are working on the SENS strategy of creating backup sources of mitochondrial parts in the cell nucleus. This research is of interest to this line of work:
A research team has identified a protein that increases the transfer of mitochondria from mesenchymal stem cells to lung cells. [The] delivery of mitochondria to human lung cells can rejuvenate damaged cells. The migration of mitochondria from stem cells to epithelial cells also helps to repair tissue damage and inflammation linked to asthma-like symptoms in mice.
"Our results show that the movement of mitochondria from stem cells to recipient cells is regulated by the protein Miro1 and is part of a well-directed process. The introduction of mitochondria into damaged cells has beneficial effects on the health of cells and, in the long term, we believe that mesenchymal stem cells could even be engineered to create more effective therapies for lung disease in humans."
Earlier work revealed that mitochondria can be transferred between cells through tunneling nanotubes, thread-like structures formed from the plasma membranes of cells that bridge between different types of cells. Stem cells can also use tunneling nanotubes to transfer mitochondria to neighboring cells and the number of these nanotubes increases under conditions of stress.