Who can make a will to dispose of their property? Who can make a will? To make a will a person must be an adult and have the required mental capacity. A will made by a minor, being under 18, is generally invalid under State and Territory wills and succession legislation. Exceptions relate to contemplation of marriage, or altering or cancelling a prior will. If the contemplated marriage does not take place, the will is invalid. The court may authorise a minor to make, alter or cancel a will.