Genet life span is a key demographic trait for understanding life history of plants. However, the longevity of clonal plants is hard to determine, especially when inter-ramet connections are short-lived and plants subsequently move independently of one another in space by means of an expansive growth strategy. In this study we estimated genet life span in the clonal pioneer species Geum reptans, living on glacier forelands, by using a projection matrix model based on demographic field data of ramets collected at two sites and in three subsequent years. We then calculated genet age structure at different population ages using multiple simulations, including a maximum carrying capacity and density-dependent mortality. Additionally, we estimated the age of the two field populations by comparing results from simulations with population structure recorded in the field. According to our simulations, more than half of the genets die within the first three decades. However, a considerable proportion survived more than 50 years and some genets even became immortal as they produced so many ramets that the risk of the entire genet becoming extinct was zero. Simulated genet age structures were strongly left skewed with many young and a few very old genets. The rather low carrying capacity was reached only after 350 years, after which density-dependent mortality started to influence genet age structure considerably. The age of the two field populations was estimated to be 250 and 450 years, respectively. Results indicate that in clonal plants, genet immortality can potentially lead to unlimited persistence of established populations. In the case of G. reptans, old populations may experience competition and increased mortality due to the ongoing succession in older parts of the glacier foreland that will prevent populations reaching their maximum carrying capacity. But due to the ability of this plant to colonize new sites and follow retreating ice on glacier forelands, populations of G. reptans can be very old as recorded here for the two field populations in the Swiss Alps.