There is information missing here. Why are they leaving? What work are you giving them it takes that long to play even? What kind of juniors are you hiring to begin with?
Your comment seems to insinuate that the blame falls with the company. There is just a long ramp up that needs to happen with junior hires. There is so much technology and processes to learn about.
Humans learn incredibly fast when motivated, so yes, I am quite skeptical when someone throws such a vague statement as if we should just accept the problems lie with juniors and not any of the many things a company could be doing wrong. In your case, "so much technology and processes", I'm already skeptical an individual needs to know that much to be productive in a junior role, for a few contexts only.
Cause all the recruiters start calling. I left my first job after 12 months, BigCorp needed developers and I was offered double then current salary.