I think the bottom line is this - twenty, thirty, forty years ago people were generally more tolerant/empathetic towards very young parents because there were far fewer opportunities for young women in education, travel and career opportunities, and it was much easier for young couples to have stable permanent employment and a home that wasn't funded soley or in part by the tax payer. It was expected that everyone bar a few very intelligent, highly educated, ambitious, (almost always middle-class)independent women (who would be viewed with a mixture of awe and suspicion, frankly) would be married with children by their mid twenties.
These days if you are under 20, or even 25, many people would be a bit baffled that you would even want to be saddled with a child so soon, when there so much else you can do first, and they might be downright irritated if you clearly couldn't really afford a child. I don't think marriage really comes into it now - but people do still like to see evidence of a solid, healthy and reasonably long-lived relationship, where BOTH parents are committed to having the child) Of course there are plenty of older mothers who fit don't fulfill all the ideal criteria either, but I suppose the accepted wisdom is that at some point you just have to get on with it and hope for the best, whereas at 20 and under, you may be considered to be rather over-optimistic at best, and downright irresponsible and silly, at worst, depending on your individual circumstances.
Of course we should not make the assumption that someone will be a bad mother, be unintelligent, with poor educational achievement, low lifestyle expectations and a poor sense of social/moral responsibility just because they are very young. Though, let's face it, women who fit that category do have an alarming tendency to have children very young, make appallingly misguided choices with their neanderthal menfolk, have no means of financial security, and a sense of entitlement/expectation that the state will sort them out. We all know that - there's no point pretending it doesn't happen. Because those young women are so profligate now, it has become easy to sterotype all very young mums.
I am (somewhat)overweight, and I get really angry about the assumptions made about the mindset/lifestyle of fat people. I can tell you until I'm blue in the face that I eat tons of salad and veg, abhore junk food and fizzy drinks, rarely sit in front of the TV,
take lots of energetic walks with my dogs, and frequently have days where I barely sit down at all until 9pm. But it doesn't change the fact that I am clearly still doing something wrong, as I'm fat. And until I am a size 12 people will continue to make certain assumptions about how they know I must secretly eat, and how lazy/inactive they know I must secretly be. A strange analogy, but it's a good one, I think!