Dementia will affect roughly 10% of Americans over the age of 65 and roughly 50% by the age of 85. Not surprisingly, dementia is most often associated as a disorder of later life, but this does not mean it exclusively affects older people.
Although dementia tends to be thought of as a diagnosis, strictly speaking it is a term used to describe symptoms. Dementia is a progressive disease, meaning that it gets worse over time. The start point for dementia is less clear and the symptoms may actually follow years afterwards. Dementia is most often associated with the development of senile plaques, which actually start around the age of fifty. Early onset dementia, alcohol induced dementia and dementia with Lewy bodies are other examples of dementia that are not related to old age.
The strongest risk for Alzheimer's is age, but dementia is not an inevitable consequence of age nor does it only affect the elderly.