How does court measure testamentary capacity?

In 2019, at the age of 96, Irlene Johnson executed a new will, leaving the residue of her estate, including title to two farms that she owned, to St. Jude Children’s Hospital, and leaving nothing to her family.  The will revoked a 2010 will that had given a life estate in a 400-acre farm to her son, with a one-half remainder interest to her grandson and one-half to a trust for the benefit of her great-grandchildren.  It had given a half interest in her other farm to her grandson, with the other half to the trust for the great-grandchildren.