Someone trying to have your will accepted for probate generally must establish that you were of sound mind and memory at the time you executed your will. Even if one becomes old, frail, and forgetful, it is difficult to get a court to regard a will as invalid. Generally, those who witnessed the will being signed will almost always say that the deceased was of sound mind, was aware of his surroundings, the day or date, who his family members were, and knew that he was signing a will. The burden then shifts to the person challenging the will to prove it should not be accepted for probate.
Courts maintain a strong presumption that a will is valid. Thus, it can be costly and difficult to prove that someone was mentally incompetent, made a mistake, or was subject to fraud, coercion, duress, or undue influence when making and/or executing the will. Even if the testator suffers weakened mentality after the will was made has no bearing. The validity of the will is only called into question should an incompetent testator want to change the will at a later date.