I agree. It does seem so terribly unfair especially when you have such great wisdom to impart to them. Age in this respect, is so unkind. It is the age old story, the generation gap.. however, people of any age are only as old as they allow themselves to be!
I don't think this only happens to Great Grandparents, it can happen to any of us, and sadly, frequently does.

Maybe you should bring this up in conversation with them and tell them how you feel, after all, they will never know unless you talk to them about how you are feeling.

Don't give up, keep smiling and I wish you all the best with this!