I had always thought my grandfather on my mother's side died at age 58. . .

but it turns out, he died in 1958.

He was, therefore, about 67 years of age.

Still not ancient by current standards, but not 58 years of age, either.

That was, FWIW, the same year my parents married.

His wife, my grandmother, died at age 89, however, which is a little younger than I remembered.  It was in 1979, which is later than I remember, which means that my recollection didn't make mathematical sense, either.  I was in high school at the time, but I don't recall it that way.

That also means that she lived long enough to see one of her children die, which I knew, and two of them fall into severe illness accompanied by mental decline, which must have been hard in the extreme to endure.