The ability to make a will involves the issue of mental capacity.
In Illinois, there is a presumption that every adult is sane until the contrary is proven and the burden is upon him who asserts the lack of testamentary capacity. In other words, everyone is presumed to have the mental capacity to make a valid will. It is up to the person challenging the validity of the will to prove otherwise.
Illinois courts also recognize that someone who suffers from some mental impairment can still have testamentary capacity. There is a case where a 74 year old woman executed a will after she was diagnosed with senile dementia and had the intelligence level of a 12 year old child. Despite these short comings, she read newspapers, was aware of and interested in current events, knew her relatives and asked about their well-being and could transact business. The court ruled that she had the capacity to execute a valid will.
In summary, Illinois law requires three things for someone to have the mental capacity to make a valid will:
He must know who his spouse, children, grandchildren and other relatives are;
he must generally understand what assets he owns; and
he must be able to form a plan in his head regarding how he wants his property distributed.