Can a will be invalid if the person had dementia in British Columbia?

This answer covers the law of British Columbia.

A will can be invalid if the person lacked the mental capacity to make it, but a dementia diagnosis is not itself the legal test. The Wills, Estates and Succession Act (WESA), section 36, says a person who is 16 or older and mentally capable of doing so may make a will. The section does not define mental capacity; the test comes from case law that BC courts apply with the section.

So the question is whether the person was mentally capable of making the will, which depends on the facts rather than a label. Where suspicious circumstances relate to mental capacity, the person relying on the will has to establish capacity, and the standard remains the balance of probabilities. Suspicious circumstances do no more than rebut the presumption of capacity and of knowledge and approval.

Capacity is separate from other grounds. A challenge may also allege undue influence, which generally must be proved by the person alleging it unless section 52 shifts the onus. Formal requirements, such as writing and two witnesses, are also separate. Because how a court weighs medical evidence and good periods depends on the facts, speak with a lawyer early.