IIRC, it's only in the Grand Prix that skaters have to choose either Junior or Senior. For all other events, they can do both if they choose. In fact, that's what Griazev did in 2004 - he won Juniors World, and was 12th at Senior. Also, Belbin & Agosto did it twice - they were 2nd and 17th at Junior and Senior World respectively in '01, and 1st and 13th in '02.
Doesn't mean its a very smart rule! I actually think it is better to do eigher Junior or Senior Worlds but if a federation wants to do it than it is their decision. Having to switch back between Junior and Senior programms within 2 weeks probably puts them in a disadvantage anyways.
But if you look at the lists for ladies as an example there are a few we just saw at Junior Worlds. There's
Barbara KLERK (BEL) who is in her last season as a Junior. But what will it really do for her confidence if she fails to qualify for the free skate 3 time this season at an ISU championships?
Mirna LIBRIC (CRO) born in 1992! wow now that is brave! She skated a personal best at JWs and I hope she can do it again. I think for her being there is what counts.
Nella SIMAOVA (CZE) did very well at Euros. She also ages out at the end of the season and I hope she does better than she did in the JW short where she just about missed the cut off.
Elena GLEBOVA (EST) has been skating senior before her injury so I really don't want to consider her as one of the "juniors".
Beatrice ROZINSKAITE (LTU) onther girls born in 1992. seems like her Federation is really pushing her but i think worlds is a bit too early for her
Merovee EPHREM (MON) born 1990. I guess being the only skater representing Monaco she gets to go everywhere. but just like Klerk I wonder what its gonna do to her confidence. Lets hope she can skate a clean programm and maybe get a personal best!
Ivana REITMAYEROVA (SVK) born 1992. She did really well at JW so I guess sending her to Worlds is her Feds way of aknowledging that!
Sonia LAFUENTE (ESP) born 1991. Being the best Spanish lady I guess it was to be expected. I just hope she qualifies and has a bether free skate that she had at JW!