I would err on the side of caution and have a letter from your parents or guardian and a working cell phone number for either end of the trip.
It is not that minors are incapable of travelling by themselves it is more confirming that there are no concerns regarding custody of a minor, or they are not running off to meet their internet boy/girlfriend - I know if my kid travels by themselves I'd prefer a phone call to see if everything is fine over a postcard from Marjorca reading "so long dad".