When you're talking maturity there are many factors involved. Bone freeze occurs around 11 months. There will be no more "growth" after that, but they will "fill out" until the age of 2 and some, beyond that. In my experience, when they are 3, they are mature mentally. Dogs will consider puppies as just that until about the age of 6 months, then all bets are off. This is when you may start to see aggression from an older dog toward the youngster. Sometimes, it will be just to put them in their place, but it can lead to injury and even death.
I just thought I'd elaborate on my first, short response.