When I started my career, I was fortunate to be partnered with an art director who had a couple years' experience. My first assignment was tv spot, and about a month into my first job, I was on a shoot. I would have been completely lost had it not been for the help of my partner.
The learning curve is pretty steep when you start at your first agency, but it will be steeper if you work with more experienced people. If you have an opportunity to partner with a seasoned vet, take it. Don't be intimidated. Don't be afraid to ask questions. Don't be afraid to look foolish.
Agencies often pair juniors with juniors. This, I think, is a mistake. Juniors will grow much faster if they have someone to show them the ropes.