Yes I have to agree.
I don�t have grandparents, I wish I did, but life isn�t like that is it. My grandparents-in-law however are a unique experience to me, and a bit of an eye opener! My wife�s grandparents (on one side) are 78 & 81, still married (61 yrs), have their own house, their own life and drive around the countryside in their own car enjoying life to the full. Sometimes I believe they are more active than many of my 20/30-something friends. On the other side, my wife�s Nan is 79 lives in her own bungalow and cycles everywhere.
They are an inspiration and I am privileged to know them. It would seem form the outside, that your son and daughter�s motives are quite selfish, whether it be financial or on the basis of their own conscience (guilt for moving away etc).
Providing you are financially secure and physically adroit, I see no reason why you shouldn�t be able to continue just as you are.