Jack - our 13 yr old Cattle Dog, constantly growls at (and bites, from time to time) Ben the 8 month old puppy. Ben is completely submissive - he is NOT challenging the Old Guy in the least - but I suspect it's a combination of Jack's poor vision and hearing (so if he can keep the "kid" away from himself, he can't be startled by a puppy appearing out of nowhere?) - AND - sad to say, I think he's suffering from dementia and reverting to his former (and really awful) ways. He's food aggressive, again. He's bitten me a couple of times as he grabs at the puppy - the good news is that he's not biting hard. Yet. And he nips people when he's happy to see them (???) I guess it's a Cattle Dog thing? It sounds harsh - but the first time he draws blood, will be the last.
I give Jack Cholodin, to help with cognitive function; Sam-e to help his liver (his enzymes were just a wee bit "off") and a joint supplement.
I don't want to imply that all dogs get weird in their old age - Ike, my beloved Aussie was a regal gentleman, always. But then, he could still see pretty good at age 14.