Most hit sexual maturity between 6 and 18 months of age, depending on the size and breed of dog
Social maturity comes later, at around 2 to 3 years of age
Any wonderful puppies may become trying adolescents as part of their sexual maturity!
Some of them may well be VERY difficult.
Some just sail through, without any issues.
This can be genetically determined, or it may reflect a lack of training. It can be a very trying time, especially if you have worked hard to train your dear little puppy, who now no longer even seems to remember SIT!
It is really important to stick to your positive principles, and be patient - much the same as you would be with your teenage son or daughter.
Be assured, the groundwork you've put in is still there!