I strongly disagree - while juniors are more than welcome to look at the PRs, if you allow juniors to approve PRs of other juniors you quickly find your codebase deteriorating.
Similarly - junior might look at your PR and see that it's ok. Senior might look at it, see that you've forgot about a thing that junior didn't know about, you omitted to test a certain condition, etc.
Again, juniors are more than welcome to look at PRs and be a part of discussion, but if you're merging, you should be wary. I'd personally be against allowing a merge if you get only a junior approval.
If you allow everybody to review every PR, it doesn't imply that you'll allow everybody to approve every PR.