Background: A population sample of people with Down syndrome has been studied from infancy and has now been followed up again at age 47 years. Methods: Intelligence and language skills were tested and daily living skills assessed. Memory/cognitive deterioration was examined using two test instruments. Results: Scores on verbal tests of intelligence changed little. Those on a non-verbal test, on self-help skills and on both memory tests showed some decline, even when the scores of those already suffering from dementia were discounted. Conclusions: At age 47, scores on most tests of even the majority of the cohort (i.e. those not definitely diagnosed with dementia) showed some decline. While this includes the scores of people who may subsequently develop dementia, it may also reflect the normal ageing process in this population.