What the causes of aging are and which factors define lifespan are key questions in the understanding of aging. Here, it is argued that cellular life involves (i) inevitable accumulation of damage resulting from imperfectness and heterogeneity of every cellular process, and (ii) dilution of damage when cells divide. While severe damage is cleared by protective systems, milder damage can only be diluted. This is due to the high cost of accuracy, the greater number of damage forms compared to protective systems, and the constraints on cellular life inherited from the prokaryotic world. This strategy also applies to cancer cells, which are particularly dependent on damage dilution. Imposing restriction on cell division necessarily leads to aging. Interventions that extend lifespan act through metabolic reprogramming, thereby changing both damage composition and the rate of damage accumulation. Thus, heterogeneity leading to myriad mild damage forms represents the cause of aging, whereas the processes that affect the damage landscape and damage accumulation are lifespan regulators.