Nigel did a pretty good job of explaining the rule, as it relates to Kansas. There is a technical exception available when the tournament is being wrestled in a split format. The split sessions are considered separate competitions for this purpose; therefore, a wrestler can compete in two different age groups if the two divisions are held in separate sessions.
A separate but related issue is the ability to wrestle in an older division than the child's actual age. The Kids division's rules for non-qualifying events allow tournament directors to combine age classes, so this is permitted only when the tournament director has expressly allowed it.