Hello, I can't answer all of your questions, because I am not certified, but I'll do my best with the rest. (I am in my third year teaching English in Japan.)
3)What are some of the specific issues that need to be dealt with based on age-group?
Elementary School - The energy level at this age level is great. They sing, dance, gesture, pretty much anything you want them to. They aren't as shy as older (Junior high, high school, adults) At this level, English is still mostly a fun subject. Also, they can pick up pronunciation very quickly.
Junior High School - I work at a Junior High most of the time, and this is a hard level to teach. I think the hardest thing is dealing with students that are so shy about speaking (pronunciation) or making a mistake that they won't get involved in the classes. This is where it is important that you use a wide variety of activities so all your students can participate.
High School - I havent taught ESL at the High School Level.
Adult - The disadvantages at this level is that its just harder for people to learn a language when they get older. But, most adult classes are great because your students are there because they WANT to learn. That is the biggest advantage at this level, and most of your students will be willing to try all your activities.
5)Based on your experience, what kind of preparation besides for certification would you recommend?
Before going into ESL, I worked with students for 8 years (since my sophmore year in high school) in a variety of areas. I tutored, coached, and was a college counselor for a bit. Just working with students is a HUGE help, learning how to talk to kids will help you in the classroom. I think many people going into education in any field underestimate the ability to talk to students, in whatever language it is.
6)How does the knowledge of the host language affect classroom dynamics
Before I came to Japan, my japanese was horrible. When I got here, my communication skills got a lot better, and not communication in Japanese, but using simple English and lots of gestures to communicate. Now that I am in my third year here, all the Japanese I know now is a HUGE help when things get a bit difficult to understand. For example, if you are trying to do a new game with your students and you need to explain something, having a bit of the native language to help goes a long way. After typing that and looking at your question again, my answer doesn't really answer your question. Sorry. But, maybe it will help in some way.
Sorry I couldn't answer all your questions, but I thought a few answers would be better than none!