Its funny how different people have different perceptions. I'm not at all keen on 'Nanny' - in my family we've aways had 'Grandmas' so that's what I wanted to be. It conjures up a lovely affectionate cosy image to me, not an old one. But my H didn't want to be Grandpa (what I would have chosen) as it sounded old to him, so he's settled for Grandad, which sounds much older to me! To be fair, he also thinks Grandma & Grandpa sound posh too (which it thy don't to me!!)
Luckily the other grandparents in each family have chosen different names so we are all happy!
Incidentally, I have always called myself Auntie when writing to nephews and nieces (and now great nephews and nieces) although they write back to 'Aunty' which looks wrong to me. Don't know why.