There are plenty of examples of 8 week old pets being trained, especially in recent years since GentleSteps training was introduced. The basic premise of GentleSteps training is to break the process into smaller, easier to teach steps. This allows Pet Stop owners to teach with very low correction levels until the pet shows some light avoidance, followed by a gradual increase in both distraction/temptation and correction levels.
The net effect is that if your 8 week old puppy is happy, hearty and healthy, you are only a week or two of training away from a contained pet.
The emphasis on treats (meat type reinforcers, not kibble!) also helps all dogs and puppies enjoy the process more.
Especially with young pups, keep the training brief and the play session long. One of the most important elements of dog fence training is significant portion of the time spent playing in the yard, particularly near the flag line.