Developed new weariness of strangers?

[mysweetkaos]

Kaos has always loved people, women much more than men. While out on walks if people ask to pet him I've always obliged (as I think seeing a nice GSD does a lot to dispel stereotypes that many have). Well as of late Kaos has decided he doesn't like people as much. For instance our vet of years....yep Kaos doesn't like him anymore and I have to muzzle him for appointments. We were headed home from a walk on Saturday and some older neighbors came out to talk to me and asked if they could pet him.....well he sniffed them, grumbled (low growl with mouth closed) and turned away from them while placing himself between me and them. Is it common for dogs to get more weary of people they don't know as they age? He is 9. Up until now I would say he has only not liked maybe 1 out of every 100 people he meets.....and it is usually someone who sets off the creep factor for me as well. Any one else had the same experience? As for the neighbor story he was perfectly fine with me standing and talking to them, he was also fine with our puppy rolling around like a spaz and getting petted by the neighbors.....he just didn't want them to touch him