Not sure honestly. I'm in my late 20's, clean cut and dressed fine and showed them a printout of my AA itinerary (which included a return flight). They looked at my passport and even asked "Why have you only traveled once internationally in the last 9 years? Me, "Have been traveling domestically in the United States instead and was in college." The one passport stamp I had was from CDG where I remember no questions were asked. I said Bonjour, handed the immigration officer my passport, he stamped it, and returned it to me without uttering a word.