Grandchildren often decide what they want to call their grandparents. I'm Nanna because my first grandchildren already had a Grannie (that's what her other granchildren called her). But my husband got named "Babar" when our first grandchild was 1 1/2 and just talking. That was handy as the other grandfather is "Grandad". Remember we are talking about titles, not names...My name is Penny but you can call me Nanna... I love being a Nanna, and all my grandchildren call me that.
I have to agree with you Alison, my dad's mum insisted in being called nannan and I found it so embarassing when a teenager. My other gran was just 'gran' my grandchildren call me grandma as though I'm a sedate and graceful old lady. I may be 62, but sedate and graceful I'm not.