"My grandfather is one hundred years old."

Translation: 祖父は百歳です。

[Ravi507280]

そふ is for my grandfather and おじいさん　is for other person's grandfather

[shadronsoul]

Sometimes I keep thinking why doesn't Duolingo let us know things like these.

[MUchiGrazi]

It does... On the written lessons it mentions it. And also on the tips (underlined words/sentences) :)

[PeterDrew9]

What written lessons? And all I see in the tips is both words with no indication of when you would use one or the other.

[AdonisCham1]

Ravi san... arigato, ne.

[Falcon198016]

祖父は百歳です。

[Vakar624235]

Grandfather was into 1 year older senpai at school.

[RayyanSheh1]

そふはひゃくさいです。

Sofu wa hyaku sai desu.

[Ben813848]

There shouldn't be anything wrong with saying the following, right?

"わたしのそふは百さいです"

Would there be a reason that it would be wrong to say "Watashi No" other than that it's implied.

[IAmEki]

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.

[Alan946894]

2020.4.24 It's gonna be redundant unless in the conversation whose grandfather becomes unclear. Even then if it's unclear, it's common for the listener to ask

誰のおじいさんですか？

if you even want it more polite

どちらさん＼さまのおじいさんですか？