The BSA Insignia Guide, in the Boy Scout Advancement section, says "Eagle Award, silver medal suspended from scroll, ... Adults wear only on formal Eagle occasions." This would seem to imply that until the boy is 18, he can wear it for any occasion, formal or not. For adults though, the Guide specifically says "Adults wear only on formal Eagle occasions". My observation for medals in general, is that most only wear their medals for courts of honor and such. But unless the term "formal" is clearly defined, I would say that whenever the wearer feels proud enough to wear the medal, then that