No, memory loss is not inevitable as we age, but we'd be very fortunate to avoid it happening. Memory starts to decline imperceptibly in early adulthood (Alzheim Dis Assoc Disord, 2003; 17: 162-7); by age 65, about 40 per cent of us will have measurable memory impairment. Recall of words and names is the most affected, leaving truly long-term memory (such as for stories) intact. For many people, it's not much of a problem, but about 10 per cent have what's known as 'mild cognitive impairment' (MCI) that, in more severe cases, can have an impact on daily life. In MCI, an impaired short-term memory prevents people from, say, keeping appointments or refraining from repeating the same story.
The most drastic types of memory loss are dementia and Alzheimer's disease, the results of two different types of brain damage identifiable on brain scans.
Evidence suggests that the risk of Alzheimer's and dementia is lower in those who have been intellectually active in their lives (Proc Natl Acad Sci USA, 2001; 98: 3440-5), although there are notable exceptions such as the novelist Iris Murdoch. Nevertheless, by and large, the higher the educational achievement in youth, the better the mental functioning later in life. Post-mortems show that college graduates tend to have a lower risk of Alzheimer's and dementia (Brain, 1999; 122: 2309-19).
One risk factor for Alzheimer's is serious head injury earlier in life (Alzheim Dis Assoc Disord, 1998; 12 [suppl 3]: S10-5). In a comparison of soccer players with swimmers and runners (athletes less likely to incur head injuries), over 30 per cent of the soccer players had impaired memory compared with less than 10 per cent of the swimmers and runners (JAMA, 1999; 282: 971-3).
Assessing the various stages of age-associated memory loss involves simple question-and-answer tests such as the Mini-Mental State Examination (MMSE), which consists of 30 items testing short-term memory. It's most useful in assessing less-educated people. A more sensitive test for aged graduates is the 'delayed recall' test, in which a list of words has to be remembered 20 minutes later.