Assuming we're talking about product managers for software/app products:
I'd say the "junior product" position is probably a frontend developer. You're making a lot of small decisions developing a frontend, and you develop a taste for it. Once you have taste, you really start contributing to the overall thought behind the product. That's when you become a reasonable hire as product manager.
As for me? Well, I was hired as a product manager after being what amounted to a "full stack" developer for years and years at a small (still smallish) startup. Having said that, it really ended up being much more of a project management role than anything else.
I'd say the "junior product" position is probably a frontend developer. You're making a lot of small decisions developing a frontend, and you develop a taste for it. Once you have taste, you really start contributing to the overall thought behind the product. That's when you become a reasonable hire as product manager.
As for me? Well, I was hired as a product manager after being what amounted to a "full stack" developer for years and years at a small (still smallish) startup. Having said that, it really ended up being much more of a project management role than anything else.