In the scenes depicted by the Memory Stone, Su Rin congratulated Saya on achieving "immortality."
Then, as a member of the Soul Clan capable of transforming Saya into a higher being, Su Rin must have already attained "immortality."
Would his appearance no longer undergo any changes?
If we consider this possibility,
that "person with the same name as Su Rin, Su Rin's grandfather" from sixty years ago,
——could it also possibly be Su Rin himself?
...After all, it's unlikely that a grandfather would name his grandson the exact same as himself.
Deducing from this,
there might have never been a grandfather named Su Rin.
The Su Rin from sixty years ago and the current Su Rin could be the very same person.
Having gained "immortality," his features would remain unchanged, naturally he wouldn't age.
This could also account for the old lady's words of "you look almost exactly like your grandfather when he was young."