This is the 92nd instalment in a series on dementia, including the research into its causes and treatment, advice for carers, and stories of hope.Perhaps it is because the symptoms of dementia generally only manifest late in life that we are inclined to think of the condition as a problem of old age. But exposure to risks that may trigger it starts years, often decades, before the first memory lapse.The idea that we should start taking steps to prevent dementia in midlife – when we reach our forties – is beginning to take hold, changing the way we think about brain health.International reports now frame dementia as a life-course condition with end-of-life symptoms, and emphasise midlife vascular, metabolic and lifestyle risks.Think of it like this: your late thirties and forties are the age when you might be exposed to a slow‑burning infection with a decades‑long incubation period, but symptoms do not show until your sixties, seventies or eighties.This explains why doctors increasingly view midlife as a window for intervention. It is during this life stage that conditions such as high cholesterol, type 2 diabetes, insulin resistance, obesity and high blood pressure tend to surface or accelerate – and each of them elevates dementia risk.