the studies mentioned above describing prevalence over the age of 65, there was a strong age dependency: from the age of 35 years onwards, the prevalence of dementia approximately doubled with every five year increase in age. In contrast with studies describing populations over 65, males seem at a higher risk to become demented before they reach the age of 65 than females. Extrapolating these figures nationally suggests that there are well over 18 000 people with dementia under the age of 65 in the UK. At 34%, AD was also the most prevalent cause of dementia among younger people, although with less prominence than at old age (fig 3). The relative prevalence of VaD (18%) roughly equals the prevalence at old age. Frontotemporal dementia (12%) and alcohol related dementia (10%) were relatively more prevalent among the younger population than among elderly populations. These figures underline the fact that, although relatively uncommon, dementia does develop in younger subjects, and it should always be part of the differential diagnosis in patients with cognitive complaints. Furthermore, these data also highlight the differences between dementia in younger people and dementia in older people, with frontotemporal dementia and alcohol related dementia being relatively common causes of dementia in the younger age group. ‘‘When I’m sixty four’’ Nearly 40 years ago, The Beatles launched a famous song which included the words ‘‘Will you still need me, will you still feed me when I’m 64’’. At that time Paul McCartney, looking at his 64 year old father, wondered how life would be at 64—considered ‘‘old’’ at that time apparently—probably also being afraid for age related diseases, such as dementia. In the coming decades, the financial and emotional burden placed by dementia on the working age population will rise notably. As the age distribution of the western population shifts, the rapid increase of the prevalence of dementia with increasing age means that both the number of affected individuals and the affected proportion of the total population are increasing. This will be especially prominent in Europe, where the median age of the population is higher than in all other parts of the world. Based on several metaanalyses of epidemiological studies and the population projections of the United Nations, the number of prevalent cases in Europe in the year 2000 was about seven million.5 Within the next 50 years, this number is estimated to more Figure 1 Causes of dementia with late onset (> 65 years). AD, Alzheimer’s disease; VaD, vascular dementia. Based on Lobo et al. 3 Figure 2 Pooled prevalence of dementia by sex. Based on Lobo et al. 3 Figure 3 Causes of dementia with young onset (, 65 years). Based on Harvey et al. 4 v3 NEUROLOGY IN PRACTICE www.jnnp.com copyright. on December 4, 2021 by guest. Protected by http://jnnp.bmj.com/ J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp.2005.082867 on 16 November 2005. Downloaded from than double to well over 16 million patients with dementia. Not only will the number of patients with dementia increase; in the same time span, the working age population will considerably decrease in number (fig 4). While in the year 2000, there was a ratio of 69 working age persons to one demented person, this ratio will decrease to 21:1 in 2050. In this paragraph, an overview of the prevalence of dementia has been given. Prevalence is determined by both the number of new cases over a given period of time, and by the duration of survival once patients have the disease.2 Death results in a decrease of prevalence; therefore, diseases that quickly lead to death may have low prevalence, even if they occur frequently, while diseases with long survival have higher prevalence, even if they occur with lower frequency. From the above it follows that studies based on prevalent cases yield associations that reflect the determinants of survival with disease just as much as the causes of disease. This can result in misleading situations—for example, if a new treatment would positively influence the course of dementia by lengthening survival (although not curing the disease), this would result in a higher prevalence. In such a