The more people I meet who teach Aikido, or practice Aikido, in the middleage to retirement years, the more we talk about diet, exercise, and losing those extra pounds that incur the "spare tire" or "beer belly" look.
Indeed, the better muscled you are in your youth, the easier it is to acquire that Santa Claus belly as you grow older.
Question is ... Can the instructor still perform, teach, and do a normal class with the students? If that is so, then I would say no matter what the physical looks of the instructor, then they have met the minimum requirements.
Of course, we are all attemping to flatten that belly, maintain muscle tone, and some resemblence of youth, but I begin to understand the amount of transition which occurs as I grow older, muscle mass declines, and the less than youthful physical fitness comes home to roost.
Most instructors enjoy being part of the class, and if they can do this with ease, I would say that is the only requirement of qualifying physical fitness.