The AKC Standard says the German Shepherd "has a distinct personality marked by direct and fearless, but not hostile, expression, self-confidence, and a certain aloofness that does not lend itself to immediate and indiscriminate friendships. The dog must be approachable, quietly standing its ground and showing confidence and willingness to meet overtures without itself making them”.
That's a great description of an ideal German Shepherd.
When you're talking German Shepherd Dog 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 our experience, when they are 3, German Shepherd Dogs are mature mentally and should be considered an Adult.