It's weakness is that it doesn't makes sense that they have no memory of their life before they turn 20.
Actually, I see that as a good thing. Can you imagine the long, detailed backstories some people would insist their characters had? And how horribly they'd clash with what had actually happened in that town? Not to mention that they'd still run into the problem of where they came from in the first place. I can just see it now, "My parents were killed by the leader of this town soon after I was born, so now I hate all authority and want to overthrow the government."
