All individuals have the right to make their own decisions, unless they are not mentally capable of doing so. However, it can sometimes be assumed that an individual is no longer able to make their own decisions because of a condition that affects their cognitive abilities when this is not actually the case.
As such, you should not assume an individual does not have mental capacity unless there is good evidence to prove otherwise. If you assume an individual does not have capacity then they could miss out on opportunities that enable them to make their own decisions. These opportunities could make them feel empowered and in control, so you should not take these opportunities away from them unless it has been proven that they are not mentally capable.