I think your teenage pregnancy example is probably reasonable, I could see that being a big turning point in a mother's life, where things began getting tougher/worse.

I guess my point is that even if it's clearly an individual-failure, how long should that condemn her to a life of struggle and hardship, let alone her children? If one step off the straight-and-narrow in society means that you need to beat incredible odds to make it back onto the happy-path, we need to work to make that easier to do. Regardless of whether you fell off that path by your own actions today, last week, or decades ago when you were a child.

I'd want to live a society where those struggling get repeated lifelines and assistance to help find some stability. I guess I don't see what the other option is; I don't think we can just give up and let them (and their children, and their children's children...) struggle and fail forever. Even if that means that they need more support/attention than the average citizen for however long it takes.