My paternal grandmother just recently turned 99 years old.
That's almost a century!
Her mind is still pretty darn good. Sure she has a few episodes of slight confusion, but for the most part, is right on top of things. She is still strong willed and determined and will let you know without a doubt if she disagrees with your thoughts or opinions.
She and my Granddad fell in love and married pretty young, when they were only 17. My dad was born when she was 19, then my aunt when she was 20.