Demographic ageing is a positive trend, as it is rooted in social development and the achievements of life. Today, the key driver of ageing is the greater population longevity. So, with demographic ageing underway, population longevity is emerging as an increasingly central notion in the framework of modern societies. The concepts of demographic ageing and population longevity, although close, do not coincide, and the fixed chronological age markers used to assess demographic ageing are not sensitive to the social changes underway, reflected in longer average life spans. This article: analyses the limits of the relationship between demographic ageing and population longevity; discusses the relevance of maintaining traditional metrics based on fixed chronological ages common to all populations, which began in the past, in short lifespans, for assessing demographic ageing; proposes the use of prospective/remaining age as an angle for analysing and evaluating the age and social dynamics underway in this Age of long lives.