I am skeptical of 100% anything, so did some looking:

>Mutations in PSEN1 and APP are associated with complete penetrance, meaning that all individuals who have a PSEN1 or APP mutation will develop AD if they live a normal lifespan [1]

and

>[Age-of-onset] Usually 40s or early 50s (range 30s-early 60s) [2]

Note that this is only for early-onset Alzheimer's, representing ~5% of all Alzheimer's.

>Our study showed that the rate of EOAD in AD is 5.5%, not 1-2% as usually demonstrated. And our results indicated that the rate in developed countries was relative higher than in developing countries [3]

[1]https://pmc.ncbi.nlm.nih.gov/articles/PMC3326653/

[2]https://www.ncbi.nlm.nih.gov/books/NBK1161/table/alzheimer.T...

[3]https://pmc.ncbi.nlm.nih.gov/articles/PMC4356853/