Microbes in the digestive system seem to have some influence on aging, insofar as they interact with the immune system, epigenetic regulation of nearby tissues, and so forth. In effect they act almost like an additional organ or biological system. Researchers are very much in the early stages of trying to understand how microbial life in the body fits in to the bigger picture of metabolism and aging - which is already very complex, and likely to become more so:

The ageing process affects the human gut microbiota phylogenetic composition and its interaction with the immune system. Age-related gut microbiota modifications are associated with immunosenescence and inflamm-ageing in a sort of self-sustaining loop, which allows the placement of gut microbiota unbalances among both the causes and the effects of the inflamm-ageing process.

Even if, up to now, the link between gut microbiota and the ageing process is only partially understood, the gut ecosystem shows the potential to become a promising target for strategies able to contribute to the health status of older people. In this context, the consumption of pro/prebiotics may be useful in both prevention and treatment of age-related pathophysiological conditions, such as recovery and promotion of immune functions ... Moreover, being involved in different mechanisms which concur in counteracting inflammation, such as down-regulation of inflammation-associated genes and improvement of colonic mucosa conditions, probiotics have the potentiality to be involved in the promotion of longevity.

Link: http://www.ncbi.nlm.nih.gov/pubmed/23079287