Comment by armadsen

There are multiple specific possible mutations that cause 100% penetrant dominantly inherited early onset Alzheimer’s. And there are three genes where mutations can cause it: PSEN1, PSEN2, and APP. The average age of onset seems to depend on the specific mutation. In my (wife’s) family, onset is mid 40s to mid 50s. Some families get it even earlier, but 30s seems rare.