I chose GRANNIE when first DGS was born, and I was 50. He is now 21 and still calls me Grannie. I remember when I was doing the "day care shuffle" when the boys were little, the day care providers were so surprised to meet me after they had heard all about Grannie. They thought I was going to be old and frail. My Gkids argue about who will get my truck when I pass into the great beyond.
They called my DH Pepop. I don't really know where that came from, just what the first DGS started saying. Whatever you choose will be so wonderful to hear the first time the grandchild can say it. It doesn't matter what he/she calls you, just know that you will have a very special place in that little heart.