Yes family history is only a guide.
And also each generation is tending to live longer than the preceding one due to better health care, better knowledge of what is bad for us etc etc.
However there is also now a Catch 22 in this as more and more people are being affected by Alzheimer's before they pass. our bodies can outlast our minds. And like you that concerns me.
One of my grandmothers was an early frequent traveller, but of the Ox and Dray kind. Born at at the turn of last century on Cape York, the daughter of a Gold Prospector, she travelled before she was a teenager by Ox and Dray to the Victorian and Kalgoorlie Goldfields amongst other places on roads that were often little more than tracks before settling in wheat-belt territory in Victoria. No First Cabin, but lots of wide open vistas and amazing tales. She was still playing tennis in her 70's and lived a rich and vibrant life outlasting two husbands before passing on at 96. Her last 6 years though she just faded away due to Alzheimer's from a wild and rich soul to a sad, scared and unhappy lost soul.