I think the most important question you can be asked is "Why do you want to coach?" If it's done right expect several more layers of follow-up whys. This is a question about your motivation that also eventually reveals your true coaching philosophy.
A young coach is going to be inexperienced in a lot of areas. That's understood in your case - it's not an issue if they've asked you to interview. As long as your motivation and coaching philosophy are well understood and aligned with the head coach's, I can't see you not getting the job.
In no specific order other possible questions:
* Do you have any other commitments that might prevent you from attending all practices and meets? If so, please share the frequency and extent to which you might miss either. (Note that it's really up to you to share details about those other commitments - it's really none of their business, but you might get some further consideration for being open.)
* What have you done in the last 12 months to prepare for this kind of role? Note that your personal fitness is nearly irrelevant to this question. Think in terms of what have you done to become a better coach of adolescents?
* Can you tell me about any prior experience you have had leading kids younger than yourself? What did you enjoy the most? What were your frustrations? What did you learn from the experience?
* How do you anticipate drawing a boundary between being their coach and being their friend? (Note that in my experience young coaches too often want the kids to like them rather than respect them. Initially it's great, but too often turns in to a miserable situation for the coach when they don't have the respect necessary to execute the role.)
* What events would you feel comfortable coaching? What is your favorite event to coach? Tell me three things you would want your athletes to know about that event by the end of the season? How do you plan to teach them those things? What problems do you anticipate in regard to them mastering those things and how will you overcome them?