Population ageing is expected to lead to significant rises in climate risks because vulnerability rises sharply throughout people’s later years. When assessing the vulnerability of older people, however, what’s important isn’t the number of years someone has lived (i.e. “chronological age”) but rather their functional abilities and characteristics; the latter is better captured by remaining life expectancy or “prospective age”. Here, we show that assessing growth in the size of older populations using a prospective rather than chronological age perspective can help avoid overestimates of future risks to climate change. Compared to an analysis based on chronological age, the projected increase in the vulnerable population share seen in the prospective age analysis is considerably lower. The differences between the two perspectives increase with age, decrease with country income level, and are larger in futures that give priority to sustainable development. Thus, while ageing certainly poses major challenges to societies facing climate change, these may be smaller than thought. Prospective age offers a relatively easily implemented alternative for projecting future vulnerability that better accounts for rising longevity.