The way I understood it is your FS test level determines what level of competition you are allowed to compete in. Why is Junior Worlds an exception? In reverse, can someone who hasn't passed their Senior FS skate go to Senior Worlds?
The US uses a testing system. (As do some but not all other national federations.)
Within the US qualifying system, you need to have passed the USFS tests for the level you want to compete at and you can't go back and compete at a lower level. These rules apply to domestic US competitions only.
The ISU does
not have a testing system. In international competition, eligibility for senior vs. junior vs. novice is determined by age only with some overlaps. (Juniors need to be at least 13 and not yet 19 as of the previous July 1; Seniors need to be at least 15.)
Federations can send any age-eligible skater they like to international junior and "senior B" events. (The senior Grand Prix is by invitation of the host federations, with some rules about who gets chosen first.)
For ISU championships (Worlds, Junior Worlds, Europeans, Four Continents), in addition to age eligibility, skaters also need to have met minimum technical scores at previous competitions at the same level.
That's the closest that the ISU comes to anything resembling a test requirement. They don't care or keep track of each and every federation's test structure or domestic competition structure. That's up to the individual federations. Some use tests, some do not. Some have age cutoffs that match the ISU age limits, some do not.
Some federations allow skaters to compete at more than one competition level in the same discipline in the same season. I don't personally keep up with the details of other countries' federations so my information might be out of date, but at least at some points Russia had a separate junior championship to determine Junior World assignments that included some of the same skaters who also competed at their senior nationals. Japan used to (and maybe still does?) invite top finishers from their novice nationals to compete in the national junior championships, and top finishers in junior to compete at the senior championships.
E.g., "Mao Asada...won the Japanese novice national championships in the 2002–03 season, and earned an invitation to compete at the junior championships, where she placed 4th. She also competed in the senior national championships and placed 7th. In the 2003–04 season, Asada repeated the same placements at the novice and junior level and placed 8th at the senior nationals."
https://en.wikipedia.org/wiki/Mao_Asada
Those examples are only possible if the championships for different levels are held at different times.
The US does not do that.
Didn't Michelle Kwan pass her Senior FS test behind her coach's back so she could jump up to Senior level competition?
Yes, domestically, in 1992-93.
She competed at 1993 US Nationals as a senior. And then she was sent to 1994 Junior Worlds (held in December 1993), which she won.
To reiterate: US rules and international rules for junior vs. senior eligibility are very different, and other countries may do things differently than both.
