You can teach fundamentals to 6 yo. I used to hand out stickers for kids that did drills correctly and rewarded all regardless with a less valuable prize. Drills: Show me your fielding stance, run through 1st base by holding out a treat past the bag, hitting the dot I drew on the ball while on a tee, going to the correct position. We had a blast. Kids are sponges and they learn from repetition. AAU starts Nationals at age 8U. The "U" stands for under and I'm sure there are many 7 yo and a couple of 6 yo that will play in that tournament. I used to have a couple of 12 yo that towered over all my other players. They were the "Studs". Now 6 years later both are out of the game becuase 1) they stopped growing 2) They thought they would stay ahead of everyone else and everybody passed them because they quit working on their game. The most important part is having fun, no matter if your 6 or 60.