They may not have a choice over that, but they may have had control over many of the decisions that led them into that situation. E.g. they could have chosen to study harder, to go back to school, not to have children when they lacked the money to support them, etc. They could have chosen to spend less money on consumables like cigarettes and alcohol.
I suspect this is particularly true in countries with socialised education/healthcare (most 'western' countries apart from the US). Most of the poor people I've met here in Australia, for instance, are either poor because a: they're happy with a low paying job, or b: they continually make bad decisions and have an anti-intellectual attitude, a distaste for learning and self-betterment. In my experience the ones who actively work towards improving their situation rarely stay in poverty for long.
>basic nurturing needed to become an independent adult in today's world
I'm not sure what you mean here. The people I've met from poorer backgrounds are some of the most independent people I know. Adversity breeds self-reliance.
This is what I actually like about current world - in this respect, it's much more fair than ever before. By far not for everybody, but literally billions of people have chance to drive their own life, within some boundaries. Every single day, life gives us opportunities, sometimes obvious, sometimes not at all.
Parents can (and do) "install" a lot of strength or weakness into their kids, but apart from truly abusive screwed up environments, once kids leave home, they are masters of their own fate. I've met people using not-so-ideal childhood as an excuse why they have screwed up life, but when looking closely, they consistently fail questions like "so why did you went binge drinking last night instead of learning a bit of that language/whatever"? Heck, even I am using my free time mostly only for leisure and fun, but I am happy and not complaining.