With the end of the use of the Explorer program within troops caused by the creation of the new Exploring program in 1959, National decided to come up with a replacement for the Explorers in the troop program. This was the idea of Senior Boy Scouts. When a boy turned 14, the same age as Explorers, he could add a strip saying "Senior" above the Boy Scouts of America strip. And that was about it. There really wasn't anything more. No special programs or advancement for this group, just the strip.
One has to wonder why they bothered.
In 1972, the idea of the Senior Boy Scout was replaced by the Leadership Corps.