My girl is 15 months old and she growled at me for the first time today and it was hilarious, yes I said hilarious. We were playing keep away/tag and her and her toy were positioned on my bed with me on the outside of it telling her "I'm gonna get it". She would move and I would reach for the toy, so she would move again, the whole time her head is down and her butt's up in the air. Play was getting pretty intense and she started growling. My girlfriend was in the other room and could hear her. She was only playing, so I am not concerned at all, there was absolutely no aggression in the growl. Just to prove to her that the toy was "mine" I did reach in there through the growls and grabbed it and I didn't even have to tell her to drop it, she just gave it to me, so I gave it right back to her. She is such a nut and keeps me laughing.