What a lot of judgmental comments. If they make each other happy then good for them. How is that "wrong" exactly? Why wouldn't they have things in common? In my office we have people from late teens up to late 60s, no one has any lack of things to talk to each other about on the staff outings.
Sounds like he was a bit depressed and she managed to press the right buttons to draw him out of it, which is great and suggests they have a good personal and emotional connection. And if, as OP says, she has had a bad time growing up, then she is likely to be mature for her age as well since hardship tends to make people grow up quicker than average. So you have what sounds like two people who've had to deal with unhappiness, making each other happy. Why anyone would be prejudiced against that is beyond me 
Maybe they'll live happily ever after, maybe they'll split up next week - just like any other relationship.
As for the teenage pregnancy angle, this happened with someone in my extended family (both parents were 18). The young mother's parents did plenty of hand wringing about her life being ruined, educational prospects cut short etc. Nearly ten years on and she's now not only an excellent mum but runs a successful business, has learnt to speak mandarin, completed a degree in Chinese studies and is about to start an MBA. The young father is doing well too and although they're no longer together they are good friends and he's a very supportive hands on dad. She says having the baby saved her from trotting off to uni to do some token degree in media studies or something else she had no genuine interest in, and getting trapped in some random career by default. She also said that a big part of the reason she had the baby (instead of ending the pregnancy as her parents begged her to do) was to get away from said parents, become independent and have the space to figure out what she really wanted to do with her life, without the pressure of being on the default college-uni-job pathway.
Frankly, our current model of bouncing young adults into degree/career choices isn't always helpful and ends up trapping them in unfulfilling careers as often as not (or saddled with debt from doing a token degree that doesn't really add much to their prospects). But that's a whole other subject. The point is, having kids early isn't necessarily a bad thing - and it means you get your freedom back in you thirties when you are often in a much better position to really enjoy it.