I would show up and communicate how extremely teachable you are - be honest about the fact that yes, you are young, but it's a benefit to them because they can teach and train you exactly how they want to. And since you're young you haven't picked up any habits that cannot be broken - and you're not set in your ways, you are moldable and willing to be flexible. Since the ball is in their court at present, have you followed up with a hand-written thank you card? This would be a perfect place to communicate all of the above - but before you do that, thank them for their time, for giving you the opportunity to interview, mention a couple of details about what makes the company such an ideal place to work, or such a team-oriented, conducive work environment, or how great the people are (note specifics), etc. :-)