Mental maturity - varies between breeds and dogs among breeds.  From the 5 Caucasians I have raised my observation is that to a great extent mental maturity is at about 2 years old but continues for a few more years.  I think that at 2 years the dog is probably 80% mature and ready for whatever nature will throw their way.  Dogs in the wild will mature more quickly where their survival depends on their abilities and not the charity of people.