I think that the available evidence is somewhat less encouraging - Alzheimer's probably starts 20-40 years before we see significant signs of it. Most of the strategies we have are building or preserving cognitive reserve, rather than stopping or reversing the destruction of neurons and glial cells. This means that the underlying progression of the disease process is more advanced by the time we see symptoms.

At a practical level, the difference between delay and prevention of dementia may not matter much. Something will get each of us eventually. If we succumb to a heart attack or lung cancer before dementia is apparent, that's probably a great thing for our quality of life.