Based only on my experiences.
Labs will behave like pups for life. Only slowing down as they get into old age.
Golden Retrievers seem to begin to age into well mannered adults around three, maybe a bit earlier.
Siberians become fairly calm at three or four, but always remain escape artists.
My German Shepherd exhibited mischievous behavior his whole life.
I tend to think staying young, as in puppy, is very dependent on their owners and training.