"The big difference between Cub Scout advancement and Scouts BSA advancement ", January 21, 2019, by Bryan Wendell, Scouting magazine, © 2019 Boy Scouts of America, extract:
Rank advancement in Cub Scouting is not like rank advancement in Scouts BSA. And that’s by design.
In Cub Scouting, advancement is grade- or age-based. It’s a rank-per-year system designed to offer age-appropriate fun and challenges as Cub Scouts progress through the program.
Cub Scouts don’t go back and work on ranks designed for earlier grade levels. They also can’t move ahead to the next rank if they finish requirements early.
In Scouts BSA, though, Scouts hold the reins. They advance at their own pace — independent of grade, age or the progress of their fellow Scouts.
For more read the article (linked above).