This isn't really suggesting that all people begin to decline at 45, generally people become more knowledgeable and better able to understand abstract things as they keep adding on the years. Dementia disorders, particularly Alzheimer's, are what is being discussed her and being able to better preempt the diseases with more warning is the benefit of this study.
Really, anyone who throws the idiot blanket over seniors (like I used to) haven't had much experience with healthy and active seniors. There's just a lot of influences convincing most people that being old invariably means incontinence, dementia, sentimentalism, bigotry, an unwillingness to learn and a death-grip on nostalgia.
Even working in nursing homes, where the least functional people tend to find their way to, I've found the most common issue to be physical impairment. This often leads to incontinence because the person could not appropriately eliminate in time, and due to both they often face depression which causes every sort of problem that we perceive to be the norm in old age.