You are in a grey area when it comes to age. I suggest going to
www.usacycling.org and looking there at the Jrs information. Ummm, maybe I was wrong. To race Jrs (as per USCF rule 1A14: a Jr is in the age group 10-18 (inclusive). To be an Espoir, your age is 19-23 (inclusive). That way you are good to go.
As for Nationals, which you would find incredibly fun and challenging, you need to be at least a Category 3 rider. Other than that getting to Nationals is like any other race, you send in your money, get reservations at a hotel, come and race.
To get more information on Nationals, this website should help:
www.parkcitycyclingfest.com.
Hope these help. If you have more questions, feel free to boardmail me. I have friends who are involved with the organization, and one associate who is offering an extended cycling camp type thing who would coach you and get you ready for Natz.
Good luck.