Not to change the tack of the conversation but according to this time line Mister is a least 13 (that's how long Harry has had him) and still VERY spry.  Either some of Harry's magic longevity is rubbing off or like Mouse he's no ordinary pet.  Given how Mouse defers to him I wonder what's the deal.  Although the Za Lord Guard say they have to defend the brownies from him so maybe he's normal and important character pet live a long time in DV.