A while ago, someone asked about how much experience they'd be expected to have to get an assistant job at an agency.
It depends. For a lot of us (myself included) an agency desk is our first assistant job. Agencies are centers for information, and a lot of people start there regardless of whether they have any intention of becoming agents. If you want to be a producer or executive, putting in a year at an agency is a logical first step. Many agents understand this and hire people with no desk experience. Still, it can be very useful to do internships or work in the mailroom (or both) so you at least get familiar with the world of Hollywood. However, I've known people to get hired as assistants without having done either of those things. (They often knew important people.)
Some senior agents will only hire assistants with experience. They might:
1. Steal an assistant from a lower-level colleague
2. Hire someone from a lesser agency, or maybe a management company
Otherwise, it's tough for them to find people with experience who want the job - because a person with a year of agency experience is probably looking for a different kind of desk - studio, network, production company, etc., unless they want to be an agent. And if they want to be an agent, they're probably going to stay at their current company and get promoted (unless it's kind of a crappy agency).
Sometimes it feels very frustrating because people only want assistants who have already been assistants. But how do you get that experience? I know, I know. You have to just keep trying, be confident in yourself and assure the interviewer that you can do the job (I'm sure you can). Otherwise, it can be useful to pick up assistant-type skills (phones, scheduling, etc.) as a PA or intern or temp of some kind. And don't be afraid of starting in the mailroom. I did it. My boss did it. You could do it too.