Yeah, no matter what it just seems to be genetic luck of the draw. Some people can go to 90s and 100s and seem spry, but then one year they just hit a wall and lose it fast, as opposed to others who decline slowly or early. You can do some things with lifestyle to mitigate it--two siblings might have very different death ages depending on diet, exercise, alcohol, etc. But there doesn't seem to be much you can do to prevent dying at 75 or 85 or 105.
Look at famous people in their 90s like Dick Van Dyke, Clint Eastwood, William Shatner. They look good now, Eastwood is gonna direct another movie. This is Shatner walking around in NYC a week ago at 94:

And see how he acts here, how lively:
If you said he was 70, it would be believable. And he might be just as good in 6 months, or a year. But 10 years from now, probably 5, maybe 2, he'll hit a wall that a 70 year old in the same shape might not hit at 80.