Why does the narrator find the thought of the grandmother having once been "young and pretty" revolting?

The narrator refers to his grandmother as being "old and wrinkled" for the twenty years that he's known her. Many people have told him that his grandmother was once young and pretty, married even, but the narrator finds that hard to believe. Looking at his grandfather's portrait, the narrator also can't believe that he was ever married to his grandmother or had children; with his long, white beard and huge turban, he looks like a grandfather, not a father.
As the narrator only knew his grandmother when she was an old lady, he finds it impossible to envisage her as anything else. It's as if she just materialized one day, already fully-formed as a wrinkled old granny. The idea that she was once a child is just too weird to take in. When the narrator's grandmother told him about playing games when she was a little girl, it all seemed so absurd and undignified. And as for the notion that she was once young and pretty, well, that's something he finds almost revolting.
The narrator is judging his grandmother's youth by her appearance and demeanor as an old woman—short, fat, and slightly bent. As he never knew her when she was younger, he has no inkling of her former life. Nevertheless, even though he can't quite believe that his grandmother was once pretty, the narrator does still believe that she was beautiful in her old age. But this was a spiritual beauty, an inner beauty, far more enduring than the fading good looks of youth.