For the purposes of this Act, a person lacks capacity in relation to a matter if at the material time he is unable to make a decision for himself in relation to the matter because of an impairment of, or a disturbance in the functioning of, the mind or brain.
It does not matter whether the impairment or disturbance is permanent or temporary.
A lack of capacity cannot be established merely by reference to—
a person's age or appearance, or
a condition of his, or an aspect of his behaviour, which might lead others to make unjustified assumptions about his capacity.
In proceedings under this Act or any other enactment, any question whether a person lacks capacity within the meaning of this Act must be decided on the balance of probabilities.
No power which a person ("D") may exercise under this Act—
in relation to a person who lacks capacity, or
where D reasonably thinks that a person lacks capacity,
is exercisable in relation to a person under 16.
Subsection (5) is subject to section 18(3).