I've been lucky enough to get an offer of representation for my first novel. Unsurprisingly, various top agents had passed on it, and the person who offered rep is relatively inexperienced (a few years in the business) and works for a relatively small agency.The most important thing to ask is "what have you sold?" And not just the young agent, the agency as a whole.
Are there any things to look out for in particular with inexperienced agents, or ways to help evaluate them given a shorter history of selling books? For example, how many sales (in PM) would you expect a good-but-young agent to have after 2-3 years of experience?
Nothing, absolutely nothing, takes the place of experience when problems arise on the path to publication. A youngish agent may not have the requisite experience, but someone at the agency should.
That means you look at what the entire agency has sold. If they haven't sold anything to a big publisher, you're going to be first. And we all know how I feel about being first, right?
The number of sales doesn't matter as much as where the books have sold (to my way of thinking.)
With a youngish agent, I'd also want to have a pretty straightforward discussion about what happens if the agent leaves. Agenting is a tough business and not everyone keeps at it.
Every agent starts small (well, ok a couple didn't but they are the exception, not the rule). Make sure they're surrounded by people who didn't stay small.