In the study, 2,284 men and women in the US with an average age of 59 were tracked for 25 years.
Blood samples were analysed at the start, the midlife test, and then again in late life, when they were about 77.
The participants did not have problems with memory or thinking skills at the outset.
Mental tests showed 502 and 832 went on to develop dementia and MCI, respectively.
The US team looked at two types of amyloid beta, known as AB42 and AB40 and the ratio between the two.
Dr Sullivan explained: “A doubling of this ratio under this threshold at midlife was associated with a 37 percent lower risk of MCI or dementia, which is comparable to about five years of younger age, and a doubling of this ratio under this threshold at late life was comparable to about three years younger age.”