Why won't my 8year cat stay in,she used to .she stays on patio.

It's quite possible that she is keeping herself cool?, or dreaming of her younger days, when the little creatures she is watching were more than a good show? My bet goes to her body temperature, cats have a slightly higher body temperature than we do, and due to their coats and tendency not to pant (like dogs) they have to keep their temperatures down by other means. This may be an indicator of illness in your cat. Check her for injuries. Look at her gums, if they are white she is fighting infection. Also her nose should be wet. If it's dry, she is not well.