"My grandfather is one hundred years old."
Sometimes I keep thinking why doesn't Duolingo let us know things like these.
There shouldn't be anything wrong with saying the following, right?
Would there be a reason that it would be wrong to say "Watashi No" other than that it's implied.
Since そふ is only used for one's own grandfather, it might sound very redundant. Maybe a native Japanese speaker would feel like it sounds like "my my grandfather" or something, I dunno.