Scouting America has recently released new policies regarding Scouts who are between 18 and 20 years old. As a 17 year-old youth transitions into becoming an adult, they are able to continue participating in Scouting America programs while awaiting processing for their adult application and criminal background check. During this transition period, they are still subject to all guidelines listed for adults.
For those youth who turn 18 during the time they are attending summer camp, a high adventure base, or a national event, such as National Order of the Arrow Conference or a Philmont trek, they are allowed to finish an event without interruptions, and they must immediately submit an adult application upon completion of the event.
If you want to learn more, visit https://scouting.org/health-and-safety/gss/gss01/ to read more about Youth Protection guidelines for adult leaders.