Ageing means increase in average age
Ageing has to do with the population composition, and is used to indicate that (the percentage) older people are rising in the population. In the near future, the Netherlands will increasingly have to deal with ageing/ageing. This is because people are getting older, but it also has to do with birth waves that have been there in history. Ageing causes problems with AOW pension but also with housing/care of the older fellow man. The CBS has a theme page with the most important data on the social and financial side of aging.