A person can make a will if they are at least 18 years of age and have legal capacity [Succession Act 2023 (SA) ss 5, 7]. Legal capacity, or testamentary capacity, means that they understand the nature and effect of making a will and can communicate their wishes.

A court or tribunal may limit a person's will-making capacity, such as where a protection order is made under the Aged and Infirm Persons Property Act 1940 (SA) or where directions are made in relation to a protected person under s 56 of the Guardianship and Administration Act 1993 (SA).

A question may be raised about a person's legal capacity to make a will if they are of advanced age or if they have been diagnosed with an illness or injury that affects their communication or cognitive ability. Where there is any doubt about a person's legal capacity, great care must be taken when making a will as it may be contested (challenged). It is best to have the will prepared professionally by a lawyer. It may also be useful to ask a doctor or a specialist to assess and record the person's cognitive ability at the time they make their will.

People younger than 18 can make a will if they are or have been married or with the permission of the Supreme Court [Succession Act 2023 (SA) ss 5, 6]. The Supreme Court may also authorise a will or an alteration where a person lacks legal capacity [Succession Act 2023 (SA) s 7].