Most agents started in editorial somehow. A few started in subsidiary rights. I was an English and Political Science major in college and was managing editor of the yearbook for two years. I attended the Radcliffe (now Columbia) Publishing Course the summer after graduation and started in subsidiary rights at S&S two weeks after that ended. I came to a parting of the ways with my bosses there after five or so months and went to Warner Books as an Editorial Assistant about a month later. Spent a little over a year there and started acquiring and editing titles while I was there. I left to go to Donald I. Fine, a small, hardcover house, where I started as an Assistant Editor/Rights Associate and was promoted after six months to Associate Editor/Rights Manager. I was there less than a year when I got offered a job as an Editor for The Berkley Publishing Group. I spent not quite a year-and-a-half there when my position was eliminated. I went freelance and read for BOMC and reviewed for KIRKUS. I also read for Jay Garon, who was John Grisham's agent at the time, and I became a Consulting Editor for the Forge Books imprint of Tom Doherty Associates. As a Consulting Editor, I acquired and edited titles for the house, just like being an editor in-house, but on a project basis. One author I was editing was a client of the Scott Meredith Literary Agency. When Scott died, three of the agents left to start a new firm, Scovil Chichak Galen Literary Agency. I approached Ted Chichak about coming on board as an agent, which I subsequently did. I spent two and a half years there and then was invited to go and start my own firm, which I did in March 1996.
I think if I had to go back and figure out a route to becoming an agent, I'd suggest a Business major with an English minor, because running an agency is more about business and negotiations than anything else.
William Morris and ICM both have "training programs" for agents. I suppose CAA does also, but they don't generally do books. If you are a college student looking to become an agent, I'd probably start with trying to get into one of those agencies as an assistant.
Z