Translation:My grandfather is one hundred years old.
I had forgotten about sobo and sofu. Personally, I've never heard it in use. People usually use baba (grandma/nana/nanny) and jiji (grandad) when talking about their own grandparents. Having said that it's not like I talked about my grandparents a lot and when friends talked about or with their grandparents it was in a very relaxed/informal setting so maybe that's why I haven't heard them in use.
They are the same. Duolingo oft sets its heart on one valid translation though and can get finicky if you try to introduce another valid translation. I've learnt it's just best to learn Duolingo's preferred translation and shrug it off as that's how it is for now - but then if it still really bothers you report it - repeatedly - every time DL says your translation is wrong. But for your answer type what DL wants you to say or you won't be able to finish the lesson :/
Even in Chinese it is rarely used. In chinese there are specific words for paternal grandfather 爺爺. And maternal grandfather 外公. These are more commonly used. 祖父 translates more as father to my ancestor so could, technically, be used to refer to a great grandfather or great great great grandfather
*If you are a native English speaker, then "a hundred" is normally acceptable. "One hundred" is always assumed. :)