The Mental Capacity Act (MCA) (2005) protects those who lack capacity and empowers them to make decisions for themselves wherever possible. It applies to people over the age of 16.
The act explains in legal terms how to assess if someone has capacity to make their own decisions, and, if the person is unable to do this for themselves how decisions should be made on their behalf.
It covers a wide range of decision making, from very complex decisions such as significant financial matters, medical treatment and wider welfare matters, to simpler decisions like deciding what to eat or what clothing to wear.
More detailed advice about assessing mental capacity and decision making can be found in the Mental Capacity Act Code of Practice (chapters 3, 4 and 5).
This website has a page on mental capacity and The MCA if you would like more information about this topic.