That's been discussed many times and no "true" definition. Common definition is 2 years or less expereince. 6U is basically considered Novice as it is not a "sanctioned" weight and typically 1st or 2nd year kids.
If you have a stud 6U wrestle them at 8U.
Here is the definition that I have on our handbook:
• Novice – Defined as two years or less wrestling experience.
• Open – Defined as a wrestler with more than two years’ experience.
Novice note – The concept of novice wrestling is to allow them to develop the basics, so that when a wrestler is ready they can compete with experienced wrestlers. Over the years some wrestling parents have left their kids in Novice longer than they should. If your wrestler is winning or placing high in every tournament they wrestle, it is time to move them to open. If you want your wrestler to become better, they need to wrestle kids better than them. Beating up on a true Novice wrestler doesn’t do your child or their opponent any good. Don’t be a trophy hunter in the Novice division!