When I started my career at Leo Burnett, I was fortunate to be assigned a great mentor. At Burnett, they called the mentors Yodas. My mentor, Dave, has a voice that sounds a little like Yoda, so that was a bonus (You can hear him in this spot for Heinz ketchup).
Greg wrote awhile back about The Mentor Effect. And as a part of Ad Age's series on the best places to work, they ran this piece by Celeste Gudas the other day. In it, she points out how important mentors are to creating a great place to work. She also points out that the more senior person can learn from the junior person, which is important.
So as you're looking for a job, maybe add to your list of questions, "Will I have a mentor? Who will it be?" The agency might not assign someone officially. If they don't, see if you can find an unofficial mentor.