The best understood activity of the enzyme telomerase is that it lengthens telomeres, the repeating DNA sequences at the end of chromosomes that form a part of the mechanism to limit the number of times a cell can divide. Average telomere length in tissues is very dynamic, a reflection of the interplay of numerous processes that lengthen and shorten telomeres or change the number of cells with long versus short telomeres. The average tends to fall with ill health and age, which is how work on telomerase-enhancing treatments started, with the aim of reversing this signature in the hope that it will improve matters.
My take on reduced telomere length is that it is a consequence of damage and dysfunction, not a primary cause of aging - though it might have further detrimental effects once it exists. The principle counterpoint to that position is that telomerase enhancement in mice lengthens life. So either I'm wrong or one of the other activities of telomerase is significant, such as interactions between telomerase and mitochondria.
This open access review is a fair summary of the arguments to try increasing the activity of telomerase in humans. Note that many of the groups most vocal on this topic at the moment are selling supplements or herbal extracts backed by sketchy or irrelevant data, the usual modus operandi in the "anti-aging" industry, and an annoyance for anyone looking for serious scientific work on targeting telomerase - so take everything that contingent has to say with a grain of salt:
The elderly population is increasing progressively. Along with this increase the number of age related diseases, such as cardiovascular, neurodegenerative diseases, metabolic impairment and cancer, is also on the rise thereby negatively impacting the burden on health care systems. Telomere shortening and dysfunction results in cellular senescence, an irreversible proliferative arrest that has been suggested to promote organismal aging and disabling age-related diseases.
Given that telomerase, the enzyme responsible for maintaining telomere lengths, is not expressed at levels sufficient to prevent telomere shortening in most of our cells, telomeres progressively erode with advancing age. Telomerase activation, therefore, might serve as a viable therapeutic strategy to delay the onset of cellular senescence, tissue dysfunction and organismal decline. Here we analyze the more recent findings in telomerase activation as a potential key modulator for human healthspan and longevity.