|MadSci Network: Microbiology|
You have asked a very interesting question. To clarify your question I will discuss prokaryotes which reproduce by binary fission (splitting) and not viruses which reproduce by a different method. I do not believe that scientists have investigated how long an individual bacterium cell can live. The most likely reason for this is, as you have stated, the parent cell and daughter cell can not be differentiated. This assumes that binary fission is symmetrical, resulting in identical offspring. We can guess how long an individual bacterium might live. Of course, for any living organism its life span will vary depending on environmental conditions. A soil bacterium may be able to live for periods over 1 year. A bacterial cell probably lives in a state of dormancy when conditions are less favorable and this dormant state may allow it to live for extended periods of time. This dormancy should not be confused with sporulation. (Sporulation is another very interesting topic regarding bacteria.) This is only speculation, and it is difficult to provide a definite answer. Scientists have always regarded bacteria as populations of cells rather than a single individual cell.
Your final question suggests one way to consider the age of a species of bacteria. As time proceeds bacteria will mutate, these mutations will be subject to evolutionary pressures. The beneficial mutations will be selected for. Over thousands of years these mutations will add up and the new bacterium will seem very different from the original species. Hence a new species has formed. Scientists can use a variety of molecular techniques to identify species which are related to each other. The scientists can then construct a phylogentic tree. (like a family tree) Using this tree one can compare which species are older than others and which species were the original ancestors. This compares to age of whole species of bacteria, I do not believe you could judge the age of a single bacterium by its rate of mutation because once again it would be difficult to distinguish that single bacterium from its progeny.
I enjoyed your question very much as it made me look at the life of a bacteria in a very different light. I hope I have helped to answer your question. If I have overlooked anything please do not hesitate to email me.
Try the links in the MadSci Library for more information on Microbiology.