With my son's case, a summer birthday. He was playing age-level through 12u - he was one of four boys that were a grade ahead of the other 8 boys on the roster. After attending Cooperstown Dreams Park with his team, he switched teams and moved up to his grade level or higher. It was not a good fit when my son entered middle school and most of the boys on his team were still in elementary school. IMO, better players that have a highly competitive mindset will lose interest in playing with younger kids on smaller fields; better players will seek out the challenge to play against older kids in their own grade and higher. When my son got to 60-90 full time in 8th grade, he shifted to play summer travel two or three age groups up (local/regional competition). When college recruiting entered the picture, he shifted again into his grad class and joined the "best possible team playing the highest level competition applicable to his graduating class". Not so much a plan set by any adults, but it was the doors that my son wanted to walk through.