just like with people, dogs are constantly maturing and there are quite a few factors that slow or speed up that process. of my 2 current males I could tell they had matured not because of things they started or stopped doing but more so how they handled stressful or stimulating situations. personalities play a big part too... my 2yr old is more mature than my other dog was at 5... for him it took losing my older/dominate female (who was more mature than both these guys by 1) to put his big boy pants on but still... personality wise he is still pretty goofy in play and a bit high strung. another thing I notice is how differently training sinks in.... that moment when things you've been working on just click and everything comes together.
so.... it's very individual and something you just "know" when it happens, somewhere between 1 and 5.