How Old is Old?

I have just finished helping my grandma applying for a US visit visa. I looked at her birthday, it is March 13th 1928. So she is about 81 years old. Can you believe that? I can't. She just stayed over at our place for the last few days. She is a little slow but amazingly still very mobile, moving up and down the stair, helping with kitchen, and still able to converse with us and my sister's kids. Sometimes I noticed that she looks bored and I wonder is she being happy now? But I think she is...