I don't know, I guess it varies. My Labrador Jake started his "terrible twos" at the age of 8 months and never grew out of it until he was 9.5 years old

We spent a gazillion $$ on dog trainers, what can I say, two of them dumped us and one got Jake to behave when she was around, but it made no difference after the training session.
I have to say I have learned a lot since then and I don't have any problem whatsoever with my Mini Aussie, Pepper who just turned 2 years in December. I am much stricter with them and I make it pretty clear who is the boss and it's not her (or her other two friends) :+ . The other two are a little over 1 year, and with all of them, I make them work for their food and I make it clear what is expected, and jumping and barking is not an option for them, nor is it acceptable behavior. I snip my finger, or touch them to get their attention and say "ahah". If someone comes in to visit, I will make them sit and wait politely, works most of the time.
What I really found is helpful is plenty of exercise, I used to go to the dog park every morning (can't right now because we have puppies and we skip the dog park because of parvo as a precaution) and walk them on a leash. So now, I walk them twice a day, make them heel and sit, and they are not allowed to walk in front of me, only at my side or behind me.
I found a tired dog is a good dog
I agree with Christine though, consistency is key!
Carola