I did a lot of travelling by myself before I turned 18, and crossed many borders. (In fact, I celebrated my 18th birthday in Morocco). It wasn't any different then than is now, with the exception of the fact that then, although the government had no problem with me travelling alone to Europe, Africa, India and Asia, navigating all the attendant challenges of travel in those days, including bureaucrats, hustlers, pickpockets, pimps, prostitutes, drug dealers, diseases, insurrections, wars, strikes, outages, failures and unexpected developments of all types (all of course without facebook, mobiles, the internet or even reliable telephone networks), that still didn't stop them from telling me I wasn't old enough to order a drink when I got home to the USA.