There is no such rule.
Many many skaters have competed at Junior Worlds after having already competed at senior Worlds previously.
Some examples: Surya Bonaly, Lu Chen, Laetitia Hubert, Krisztina Czako, Zuzanna Szwed, Tanja Szewczenko, Lucinda Ruh, Julia Lautowa, Julia Sebestyen
Jamie Sale went to 1994 Worlds and Olympics in pairs and then to 1995 Junior Worlds in singles. Shelby Lyons went to 1996 Worlds in pairs and then back to 1997 and 1998 Junior Worlds in singles. Sale was between partners at the time, whereas Lyons was doing both disciplines simultaneously.
How about some men? Again, just a sampling, not an exhaustive list:
Phillippe Candeloro, Patrick Meier, Ivan Dinev, Zhengxin Guo, Neil Wilson, Kyu-Hyun Lee, Kevin Van Der Perrin
I haven't looked up pair and dance examples. Zhang/Zhang, Belbin/Agosto, and Hoffmann/Elek come to mind.
I thought I had heard there was a rule that prohibited skaters from returning to juniors after having medaled at senior Worlds even if still age eligible, but I can't find it in writing. I don't know of any counterexamples to that one.