The most convincing piece of evidence that telomeres are important in aging is that lack of telomerase (the protein that extends telomeres) or otherwise shorten

The most convincing piece of evidence that telomeres are important in aging is that lack of telomerase (the protein that extends telomeres) or otherwise shortened telomeres causes progeroid (premature aging) disorders. But there are problems. First, telomerase is activated in many cancers to allow cancer cells to divide indefinitely, so there is a fear that adding telomerase as an anti-aging treatment would increase cancer risk. It is assumed that the finite length and existence of telomeres is an anti-cancer mechanism by cells. Secondly and even more problematic, aging occurs in tissues, and aging-like gene expression signatures in cells, which still have ample telomeres. There is a lot of evidence that other things like inflammation and metabolism, which have no direct connection to telomeres, are critical to aging. So telomerase/telomere lengthening is exceedingly unlikely to be a "silver bullet" for aging. Experiments show that it can extend lifespan in mice modestly. Conversely, though, upregulating or downregulating many other proteins which have nothing to do with telomeres can also increase lifespan. It is possible that telomeres are important in stem cell maintenance, since stem cells have to last a long time and replicate repeatedly (stem cells themselves are a very hot topic in aging right now).