While revising one book and working on a new one, I've been paying a lot of attention to other people's agent searches. Especially to the searches that are for a second agent. One thing I notice is that people who are on a second agent have not necessarily parted ways because the agent couldn't sell their first book. Maybe the agent had a YA romance connection (because they normally sell adult romance), and when the author wanted to do a YA historical mystery, the agent was out of their element. In other words, the range in which the author and agent overlapped was very small. One thing I have heard more than once is that an agent, even a good one at a good agency, needs to have connections in the genre you write in. Which means that you can have an awesome bigname agent in adult lit, but that doesn't mean they can sell a midgrade novel. Or even YA.
I'm willing to allow for first-timers because everyone has a first time. But I still think that even if you're an agent who's decided to branch out into the YA market, you have to know something--and love something--about that kind of book. It HAS to be more than seeing dizzy dollar signs after the words "Harry Potter." An agent has the right to ask how serious an author is about their career and where they see themselves in the publishing world. But I think an author should also find out where that agent's heart is. What their favorite books in your genre are, what their favorite recent books in your genre are, why they like yours, and where they see you and your work fitting into the market. I know that is something important for me as I research agents. Just like the form rejection you see floating around, you can't just like the genre--"in this business, it's gotta be love."