We run younger kids at the same time as adults in our racing. The deciding factor is their ability on track, dealing with faster karts passing them, and doing the right thing when things go wrong (breakdowns, spins, etc). In fifteen years we’ve been doing this we have had one kid that we asked not to come back until he had more experience. The kids we’ve had start with us at 11-12 years old are usually at least 100 pounds. The weight make a big difference in their ability to handle the kart while driving. It also give them the endurance to last for a race that is at least 15 minutes and a couple of times a year a full hour long.
IMO one of the problems with Rookie, Cadet, and Junior classes is that they are based on age when they should be based on driver size/weight for the younger set. You get a huge variation in the weights of kids between the ages of 5 to 12. They may have the skill needed but if they can’t turn the wheel they are not ready yet.