Well... speaking as an almost 70 year old... I'd just say that it's really an unknown. You won't know what your grandparents' reaction is going to be until you tell them. And then it's too late to take it back.

I've struggled with gender identity disorder since early childhood. And I never breathed a word of it to my parents let alone my grandparents. And, personally, I'm glad I didn't. I can just hear what my father would have said: "Jesus bud, what the hell's the matter with ya?" Personally I would just keep my sexual orientation to myself unless there's a particular reason you need to tell them. But that's just me. Good luck!