In order to make a valid Will, the testator (a person making a Will) must be 12 or over when they sign the Will.
The testator must have the requisite mental capacity when they signed their Will. This testamentary capacity has been defined as 'soundness of mind, memory and understanding'.
This requires a testator:
There is a presumption that, unless the contrary is proved, the testator is sane, so the burden of proving that the testator lacked testamentary capacity falls on the person who seeks to prove, after the testator's death, that the Will is invalid.
If the Will appears rational, then that helps towards establishing that the testator did have testamentary capacity.
If there is any doubt as to whether the testator had the necessary testamentary capacity to make the Will, it may be advisable to arrange for a medical practitioner to examine the testator at the time the Will is to be signed to verify their mental capacity and to have that medical opinion recorded. It will also make practical sense to ask the examining practitioner to witness the Will.
A Will must not be made as a result of either the undue influence or the fraud of another person.
Undue influence means coercion; i.e. the testator is coerced into making a Will (or part of a Will) which he or she does not want to make. Generally, considerable pressure must be exerted before there is any chance of a finding of undue influence.
Fraud consists of deceiving the testator. For example, it would be fraud to tell a testator that a potential beneficiary had done or said something of which the testator would disapprove, when he or she had not.
Generally, a testator must know and approve the contents of his or her Will at the time that he or she signs it.