I also include junior developers who don't know much about the application or environment in code reviews. It's a good way for them to learn the code base and see what type of defects the senior developers point out.
+1 to that one. Code Review should be Peer Review - it is not about a Senior reviewing a Junior it is about a developer reviewing the work of another one. The junior's questions may actually be as useful as advices (notably by forcing you to explain your choices).
I like to have one junior and one senior developer along with the software architect review another developers code. This way you get proper code review from the senior, and the junior learns and get exposure to other peoples code. But three people reviewing is actually already a too large team for review imo.