What happens if the person who wrote the will had dementia?

A will made by someone with dementia can be challenged and potentially invalidated, depending on whether they had mental capacity when they signed it. Courts usually look at whether the person understood what assets they owned, who their family members were, and what they were actually signing. If there's evidence they were confused or easily influenced during that time, the will might get thrown out and an earlier one used instead, or the estate gets divided according to state law. It's messy and expensive to litigate, so having medical records and witnesses who can testify about their mental state at the time makes a big difference.