I have an Akita (that are known to be dog-aggressive) whom I have been taking to a dog park since she was 5 months old. I am a huge advocate of the dog park and credit it with making her so social. She is just over 3 years old now. We still take her and she is still great with the other dogs. I have heard (keep in mind that this comes from those that think the dog park is an awful place) that around three years of age some dogs will start to change their disposition and they may become more dog-aggressive. I would be so sad if that happened and so far I haven't seen any sign of it, but that was something I was told. I was being warned to not let my gaurd down as things can change. (???)