It does say that here, in CMU’s application instructions on page 7:
Early Decision also isn’t available if you’re under the age of 17 or if you’re applying Early Admission as a high school junior. (note they do have an Early Admission plan for HS juniors on the same page)
https://www.cmu.edu/sites/default/files/pzh-nqzvffvba/2021-09/cmu-application-instructions.pdf