If you are being paid to teach sailing aboard an auxiliary sailboat, then you must have a USCG license
. If you are teaching aboard a sailboat that has no engine
, or the students are not paying for the lessons, then you don't need a USCG license
. This applies in all cases, regardless of the age of the students.
You can find a good flowchart that explains all of this here...
Become a Sailing Instructor - American Sailing Association