Joe, you forget that in the ISU, Junior is defined by AGE. In the USFS, Junior is defined by SKILL LEVEL. US Senior skaters have reached and gone beyond the skill level for Juniors, but they still meet the ISU age definition. Rachael is 15, and Mirai and Caroline are 14. By ISU standards (at least 13 but under 19 on July 1, 2007 for Singles and Ladies in Pairs and Dance, and under 21 for Men in Pairs and Dance), they are all on the young side. Tommy Steenberg was 19 in November, Bianca Butler was 19 in September, Joe Jacobsen turns 21 tomorrow, Emily Samuelson is 17, and Evan Bates turns 19 in two weeks. BTW, all of these skaters except Zhang competed in the JGP this season.
Most federations send their best age-eligible skaters, not necessarily their Junior champions.
Canada almost never sends their Junior champion to Jr Worlds. This year, they are sending Myriane Samson, who turned 19 in September.