If the back story is not dramatic, there's no question; you have to let it leak out during the story when things are happening. If it is dramatic, there are questions of story cohesion -- no matter how dramatic the events of the traumatic childhood were, do they directly relate to overthrowing the Evil Dark Lord? ("Of all plots and actions the episodic are the worst. I call a plot 'episodic' in which the episodes or acts succeed one another without probable or necessary sequence.") Then, can you make them relate to overthrowing the Evil Dark Lord? Closely. With cause-and-effect. Like, the Evil Dark Lord got your father to make magic for him and then killed him, and you both want revenge and know how the magic works.
Then, you hit the second problem, which is whether they are better as on-stage or back story. Starting with the beginning of the action handles your exposition problem and raises plenty of situations rife with dramatic irony, though it does have the danger that your hero may not be in action immediately but reacting. Starting with a good chunk of the action already completed gives you a chance at intriguing mystery in your characters, and sets up dramatic revelations, but you do have to pick a logical location to start, one in which the questions will not leave the reader frustrated while you put in action enough to interest them in the back story. Do not -- repeat do not -- start with something dramatic having just happened, so we have to get backfill even to understand the first scene. Plus, there's always the danger that reader demand will send you back to write the prequel. Which can be problematic. (Since I've already rambled on about that here you can just go read that. 0:)
Then, you don't need to start all your characters off at the same place. One of the advantages of the newcomer is that the story is new to him, we can get it from the front without all that back story issues, but the characters around him can be jam-packed with back story, being considerable more media in the res.