A German Shepherd does not reach its full physical or mental maturity until after 2.5 - 3 years of age. (vonziugermanshepherds.com)
Not all children reach the same level of physical or mental maturity at the same age and there are of course, special cases as mentioned before. (mommystroller.com)
Some do not attain mental maturity until they are three or four years old — another characteristic which many fanciers consider to be part of the charm of this breed. (midwestigrescue.com)