i am not sure if different sized breeds go thru it at different ages. generally for the large breeds it is 10 months to 18 months. they generally start testing you for pack rank, seeing what they can get away with, and sometimes all their obedience goes out the window. their attitude can change towards other animals in the house also. now i will say it is varied amongst different breeds and different individual dogs. my golden mix never went thru this obstinate stage yet the golden next door is very dominant. my chow mix was very dominant from day one at 2 months of age and though he has come along way he still will test his muscles now and then and he is 6 yrs. old. we missed the teenage years with our pyr, and so far scout is pretty easy going, doesn't try to dominate us but she is only a year. hope this helped.