We will "try" them on what we call puppy sheep (very dog broke, willing to go with the flow) at about 6 months of age. They will not likely be mentally ready for training until they are 10-12 months of age. They need to be mentally strong enough to take corrections when they are not right but still willing to come back to work after the correction. We still are responsible to protect our sheep from abuse by inexperienced dogs. They are allowed to work when they are doing so correctly, they are corrected when wrong (by blocking them from working the sheep and forcing them away from the sheep), and after the correction they are rewarded by being allowed to go back to working the sheep.
They don't really learn by watching or following trained dogs.
We awaken their instinct to go to balance (place on the other side of the sheep which will turn the sheep towards us) and then use their desire to go to balance and to please us to teach everything else.