The 2003-2004 IDSF Rule Book is available on their web site at
http://www.idsf.net . Click "Statues & Rules" on the left hand side of the page, then look for the link that says "IDSF Competition Rules (Status: Barcelona 2003) (.pdf)". Also see, on the same page, the link that says "Age Groups in 2004".
Unfortunately, going by IDSF Rule 11 parts 1 and 2, you have been considered to be 16 since January 1 2004 and therefore are no longer eligible to enter any Junior II events at all.
I think your next step should be to contact your country's dance sport governing body. They are the ones responsible for nominating who gets to go to the World Championships, and will be able to give you a final ruling. According to the IDSF, when World Representatives are selected, each competitor's age must be sent to the IDSF -- presumably so that age rules can be verified.
In the USA, the person to contact would be the VP of DanceSport, Gary Stroick. You might also want to contact the USABDA DanceSport Delegate, Turtle Brennan. You can find out how to contact them by going to:
http://usabda.org/home/our_organization/officers/index.cfmAs a side note, a while ago I was talking to someone about something related to this. The person I was talking to noted that having the USABDA Nationals in August causes problems with age groups when it comes to selecting World Representatives, and that perhaps someday USABDA Nationals should be moved to early in the year to fix this problem and others like it. Another problem that has cropped up is that, because of how the dates of some of the World Championships relate to the date of USABDA Nationals, it is sometimes possible for a USABDA National winner/runner up from one year to go to two World Championships whereas a winner/runner up from another year doesn't get to go to any. It's confusing, crazy, and rather unfair.
Please contact Gary & Turtle immediately to get first-hand information straight from them.