The issue is, where are they going to live. The relative that they are living with, needs to be aware of their plans. When you are older, having to be on the same body clock as a baby/toddler is tough going and it shouldn't be inflicted on anyone.
"i’d never try to dissuade someone from having kids young. Have the DC in your early 20s, then start your career in your 30s when they start secondary school, and carry on right through without having to take a break"
That doesn't mean that you don't work, it means that for the young Women who want children and don't have the desire to go to Uni, they can do that later on. Meanwhile they just work a job that gives them the life balance they want. For the same level of debt as a Degree, you can buy starter homes Up North. You're doing the same thing, just the opposite way round.
I had my first at 18 and then had secondary infertility. if I hadn't I would have wanted my family complete by around 23. We owned our own home and my DH could support us fully. It would have been a case of tough, if he couldn't, the benefits wasn't around then. I didn't know what I wanted to do as work, but I wanted children more than anything, so it made sense to get on with it.
The level of possible immaturity, I don't think is damaging to the baby. By the time the child is five, a lot of growing up will have been done. The level of open thinking and less worry when your younger, is a bonus, as well as the energy levels and the physical side. If you've met the person that you want to be with, then you aren't missing out on anything.
It's swings and roundabouts. I worry more about young Fathers not fulfilling their role, than a young Mum.
As for the poster who sited her relative who has SS involvement, the one thing that surprised me when I became a CP SW was that my client group wasn't Teens. There were an equal amount from every age group. It was the Teens that tended to turn things completely around, though.
My Mum had me late, so going through the Menopause having her to look after and then have a young child, as well, would have pushed me over the edge.
There are some sections of Society that may be "practically children" at 19, but others will be more sensible than some 30 year olds out there. We put young Women down, in our Society, terribly.
Just because you are younger it doesn't mean that you lack what is needed to successfully to raise a child and being older doesn't necessarily give you that.