An assistant Scoutmaster is an adult leader over the age of 18 who helps the Scoutmaster deliver the promise of Scouting. Each assistant Scoutmaster is assigned specific program duties. They can serve the troop by guiding a particular patrol to which they’ve been assigned, advising certain Scouts who’ve assumed a position e.g. the quartermaster or OA representative, or fill in as necessary to assure the junior leaders have what they need to run things.
Refer to Troop Leader Guidebook – Vol 1, Page 83-84
They also provide the two-deep leadership required by the Boy Scouts of America . An assistant Scoutmaster may be 18 years old, but at least one in each troop should be 21 or older so he or she can serve in the Scoutmaster’s absence.