I have a Border Collie/Blue Heeler mix and he is the best dog I have ever owned. I got him when he was 9 weeks and he is now 3 1/2 years old. I have always taken him everywhere and he has been around people and children from the beginning. I have taken him to schools and soccer practice and he alway draws a large crowd of children because of his frisbee skills. Lately he has been barking and growling at anyone that approachs our home or car, but the worst of it is that he got nippy all of the sudden with someone. Is this a behavior that is coming out with age or possibly breed? Nothing has changed in our lives, he is with me all day, everyday and he is getting more than enough excerise, I am just not sure what is going on. Any ideas?