My nine year old tabby has over the last few months had a dirty ear. He's always been a bit grubby (unlike his perfectly groomed brother) and his ears have never been pristine. However, lately one ear is worse than the other and has been getting the black dirt-type wax that they get in the outer folds, so I've been gently cleaning it with cooled boiled salt water. His ears aren't bothering him at all, there is no odour and he's not pawing at them or showing any other symptoms at all so I'm sure it's not an infection or mites. Any ideas? Is my boy just getting grubbier in his old age?