The problems of population ageing are challenging and growing worldwide. However, es-pecially in developed countries, major changes are taking place in the survival, health and life style of elderly people. Those changes should revise the criteria by which the elderly population is ranked and the way in which to look at seniority and oldness. The paper deals with these changes as they are discussed in the literature and analyzes their recent evolution in Italy in a stepwise-process approach of ageing. The Author concludes that a person is elderly if society considers him/her to be so, especially regarding his/her social and economic contribution; he/she is an old person if he/she feels himself/herself old. Both definitions are variable not only among individuals, but also in the mass of old people, so that the relevant thresholds have progressively moved forward. The real challenge of population ageing is to carry on "ageing" the thresholds of seniority and oldness, thus recruiting more and more people now defined as elderly or old to a profitable role for society and supporting aged people towards an active and healthy seniority and oldness.
Keywords: Population ageing; age group; elderly population; old population; individual ageing; social ageing.