AEF0217 in Participants With Down Syndrome

The goal of this clinical trial is to identify if AEF0217 show an improvement in adaptive behaviors (daily life activities) in adults and older adolescents with Down Syndrome. It will also learn about the safety of AEF0217. The main questions it aims to answer are: * Does AEF0217 improve the daily life activities of the participants after being administered daily for 24 weeks ? * Does AEF0217 improve fluid cognitive function (cognitive abilities that do not depend on prior knowledge) and the crystallised one (knowledge acquired through one's culture, including verbal ability and social knowledge), the quality of life and sleep of the participants after being administered daily for 24 weeks ? * What medical problems do participants have when taking AEF0217? Researchers will compare 3 doses of AEF0217 to a placebo (a look-alike substance that contains no drug) to see if AEF0217 improves adaptative behaviours in people with Down Syndrome. Participants will: * Take AEF0217 or a placebo every day for 24 weeks * Visit the clinic 6 times with their caregiver for checkups, performing tests on a tablet and answering questionnaires. * Be called by phone at home 5 times to check that they are well.