Under Youth Member Behavior Guidlines - Unti Responsibilities it states:
Each Cub Scout den and Webelos Scout den and each chartered Cub Scout pack, Boy Scout troop, Varsity Scout team, and Venturing crew shall have one leader, 21 years of age or older, who shall be registered and serve as the unit or den leader. A unit leader may not serve simultaneously in any other position within the same unit. The head of the chartered organization or chartered organization representative and the local council must approve the registration of the unit or den leader on the appropriate form.