Well, I can see a fighter being reasonably fresh off the farm. He may stand out from the next-door neighbour's boy, who doesn't have the same hand-eye coordination or wanderlust, and spent his youth learning more about cattle whilst the hero was bunking off and learning to fight, but he's still a country-boy at heart. Besides, in a point-of-light setting, I don't see it as particularly uncommon for peasants to figure out what to do with the business-end of a pitchfork, or for a boy to learn how to handle the family sword. What would be more uncommon would be the desire to go exploring the surrounding darkness, for whatever reason.
Why couldn't a wizard be a freshly-released apprentice? I think it would be more unlikely for a wizard to release an apprentice before he was ready to face the world and stand on his own two feet.
The street urchin is perhaps slightly more difficult, if you're looking for a 12-year-old character. Then again, what's stopping an acrobatic young lad developing a handy knack of sticking bits of metal into other people's vital organs?
Why exactly is it so hard to accept that, whether through training, instinct, fate, or sheer blind luck, a 4e character couldn't be just that bit tougher than your average bear, allowing them the chance at greatness?
And are they really head and shoulders above everyone else when they start out? From what I'd heard, traditional low-level fodder such as kobolds were proving an interesting and challenging foe. Yeah, the PCs have a higher power level than level 1 3e characters, but it seems the monsters are higher powered too.