1. It depends on your goals and your experience trialing a dog. Those experienced with training and trialing may trial a young dog in classes such as jumpers and tunnelers to "test" how well they hold a start line, how well they mentally function in the environment and how well they can handle the end of a run (i.e. do they come back to the handler readily). There are no expectations ofQ'ing and often a short course is what is taken rather than the entire course.
2. I think that those who are newer to trialing and training should wait until their dogs can do complex sequences in class well before even considering trialing. Ideally they wait until the dog is able to do 15-20 obstacle sequences well in class and can handle run-thrus and fun matches well. By well I mean that the dog can successfully complete all of the obstacles and the handler is able to perform front and rear crosses when running their dog.
When you make the decision to trial your dog you are making a huge transition. You are now entering an environment where the dog needs to understand delayed gratification. I teach my dogs "yes!" which means you are brilliant but keep working which almost always makes my dogs speed up. If you go from always feeding from your hand or bait bag in class at a high rate and then go to a trial ask for 14-15 obstacles in a row with no reinforcement you are going to lose your dog's attention out there. The concept of "bridging the gap" which I've talked about is key to transitioning from training to trialing.
Remember that every time you run your dog in a trial you are "practicing" behaviors. If your dog is missing contacts in a trial, running around obstacles in a trial, knocking bars in a trial or missing weave poles in a trial then they are "practicing" all of those behaviors. Dogs are very context specific in their learning which is both a good thing and a challenging thing. Dogs can very quickly become "ring wise" meaning that they can learn to do things like miss contacts, avoid weave poles and/or breaking start line stays very fast in a trial setting. No matter how much we reward these behaviors in training if the incorrect behavior goes uncorrected or conversely the good behaviors in a trial go unmarked/unrewarded then dogs will go the path of least resistance in a trial.
Trialing can become addicting because you may be able to earn a qualifying ribbon and/or a placement ribbon when your dog did not perform the obstacles as well as you had wanted. For example if the dog puts a foot in the yellow and jumps off but you taught the dog to go to the ground with two front feet on the ground and two on the board, the judge will not fault your dog's performance. However your dog has practiced a new behavior that is different from what you had originally taught. This new behavior can soon deteriorate into a dog missing the contact and/or a dog who is launching ahead to the next obstacle (which may be an off-course) without waiting for direction from the handler.
When a handler allows a dog to "practice" these new trial behaviors then a number of things can happen. In all venues the lower levels allow more time and very often more faults in order to qualify. If you are qualifying with a lot of faults at the lower levels it will make qualifying at the upper levels more difficult. In every venue it becomes more difficult to qualify as you move up in the levels. The times will get tighter and you will need to have fewer and fewer faults to qualify. So it will feel difficult to move up to the upper levels because the dog has practiced these behaviors in a trial setting which may not be good ones to have practiced to be able to qualify at the upper levels.
My goal with my students is that their first early trialing experience is successful and fun for both handler and dog. Not everyone wants to trial but for those who do I'd like it to be a positive experience from the start.