Dementia comes on slowly and is the result of physical processes in the brain (that are only partially understood). Melinda's death no doubt added stress and may have worsened his symptoms, but a diagnosis of dementia usually comes after months or years of slowly increasing signs that something is wrong. Here's another article. I hope Brian is being well cared for. After all he's given the world and the struggles he's had, that's the least he deserves.