Objective The objective was to find out whether changes in personality and adaptive functioning or memory processes decline first in ageing people with Down syndrome. Methods We measured these variables cross‐sectionally in a Dutch sample (22 to 62 years of age) of 68 institutionalised people with Down syndrome. Results The scores on all the variables except one of the temperament scales were found to decline gradually with increasing age, but deterioration of episodic memory started earlier. Conclusions We argued that a subset of our sample suffered from dementia. Furthermore, the data suggested that immediate memory impairment is one of the earliest signs of the disease in people with Down syndrome, just as it is in the general population. Copyright © 2016 John Wiley & Sons, Ltd.