0514 How old is my grandmother?

On my family members' way to my grandmother's house yesterday,my fiance's mother asked me how old my grandmother was.I did not know.I answered,"Well,I do not know for sure,but I know she was born at the year of pig."Then I started to calculate because I was born at the year of pig also.All that I had to do was to add the number of my age with mutiples of 12.
Then my third sister's daughter,a six years old girl,who was also born at the year of pig said,"This is a easy question,she is six years old."She did not understand  why her great grandmother was ninety years old but not six years old since they were both born at the year of pig.