The standard statistical caution for these kinds of screening tests is especially important here, because while Alzheimers drugs may be more effective earlier in the disease course, none of them are "effective" in the sense of meaningfully staving the disease off; the upside to early detection is not very strong.

Meanwhile: the big challenge for screening tests is base rate confounding: the test needs to be drastically more specific the lower the percentage of the cohort that truly has the condition is. Relatively low rates of false positives can pile up quickly against true positives for conditions that are rare in the population.

The bad thing here is: you get a test suggestive of early-onset Alzheimers. It could realistically be the case that the test positive indicates in reality a coin-flip chance you have it. But that doesn't matter, because it will take years for the diagnosis to settle, and your mental health is materially injured in the meantime.