ok, heres a question. They say that a dog "matures" at 2 or 3 years depending on size, breed etc, but what does this mean in terms of behaviour? obviously it is physically as big as it will get, and has grown out of all the puppy madless long ago, so what can happen at "mental maturity"?

Well that depends on breed as well -Golden Retrievers for instance will often NEVER lose that puppyness the have, or at least not before the age of 8. :D Some other breeds do become generally more sensible and not as boisterous.

Marianne

It will often also be the age when they will establish their place in the canine pack, up until about 2 or 3 they are automatically at the bottom, at least here they are :D 

You may notice them flexing their muscles a bit.  For example my almost two year old has taken to waiting to go to her food dish until the others have nearly finished theirs and daring them from a distance to try and put their noses in hers, whereas to keep them from doing so as a puppy she would have had to have her nose in the dish :D 

As soon as I picked up on this recent game, I took her dish up, so she soon learnt that I wasn't going to have these mental battles.

This is often the age where two dogs (similar age, litermates) of equla or similar status will start to fight.