The CAA regulation states that all persons seated at a self help exit must be briefed. With BA this is pointing out they are in an exit row, asking them to familiarise themselves with the instructions on how to use the exit, if they have any questions to ask the crew and if they are happy to be seated there.
So basically if the 80 year old passenger felt she was capable, she is ok to sit there. If any crew member disagrees, then she would be moved.
It is possible a discussion took place to get a second opinion with another crew member out of sight.
PS: GLA CC, The CAA does not regulate you have to be 16 or over to be seated in an overwing exit. Providing feet are touching the floor and they can lift the hatch out, they are entitled to sit there. They can be 12, but if they're tall enough and strong enough, its fine.