Most older adults do not suffer cognitive impairment. Around 30-40% of adults over 65 have the type of cognitive loss we regard as a normal consequence of age — a measurable (but slight) decline on memory tests; a feeling that you're not quite as sharp or as good at remembering, as you used to be (age-related cognitive impairment). Around 10% of adults over 65 develop mild cognitive impairment (MCI), which does impact everyday living, and is a precursor of Alzheimer's.
There are significant differences in prevalence as a function of age. For example, in the U.S., a large sample found MCI in 9% of those aged 70 to 79 and nearly 18% of those 80 to 89. Prevalence decreased with years of education: 25% in those with up to eight years of education, 14% in those with nine to 12 years, 9% in those with 13 to 16 years, and 8.5% in those with greater than 16 years.
Large-scale population surveys of mild cognitive impairment in the elderly have produced large differences in national levels, ranging from 10% to 26%.
Although women may decline at a faster rate than men, prevalence of decline may be greater among men. For example, a large Dutch survey of those aged 85 and older found more women than men had good memory (41% vs 29%) and mental speed (33% vs 28%), despite the fact that more women than men had a limited education.
However, severe memory problems in the elderly have become more rare. The main reasons seem to be better physical fitness (partly due to better healthcare), higher levels of education, and greater personal wealth.