Ultimately I think it boils down to the fact that some women are never going to be good mothers, weather they have their first DC at 15 or 40. If they lack maternal instinct, drive, love for their children, the ability to keep a home clean and resonably tidy, the desire to read to their children or take them on walks in the woods/local park etc, all activities which are free and do-able by everyone.
The think is that often the women who would struggle to be mums at 40 have their first children as teenagers anyway, because of a whole host of socio-economic issues. If you live in an area where a committed, long term adult relationship is virtually non existant you won't strive for that, if no-one goes to school/college/university after the age of 16 you might not think about that, if having a baby at 16 won't affect your live chances in any way compared to you having a child at 26 or 36 etc why wait 10/20 years?
I'm educated, intelligent, and, (through no fault of my own - I know it seems to be a dirty phrase) thoroughly 'middle-class'. When I found out I was pregnant no-one could believe I was keeping my baby, my headteacher and form tutor both tried to gently persuade me to have an abortion or give DD up for adoption, the midwives I met all assumed I'd left school at 16 with no qualifications, one tried to tell me about a course at the local FE college for young mums to do entry level english and maths and a child development GCSE she nearly fell off her chair when I told her I had 9A* and 1A at GCSE and planned to go to University. It was assumed because I was young I wouldn't even contemplate breastfeeding. I was offered free 'quit smoking' classes yet I've never even held a cigarette! I was frequently spoken down to and patronised by proffessionals who I'm probably just as clever as, if not more so.
The only times I was spoken to as if I had two brain cells to rub together was when my parents (a Dr and a nurse) accompanied me to the appointments, they always started with the health care worker talking to my parents before realising from my contributions I actually understood what they were talking about!
I think that I have coped fine having a child so young and while its not the age I'd have chosen to start my family being so young had its advantages, I had a 2 hour labour and left hospital 24 hours later in my size 6 skinny jeans, (I doubt that would have happened if I'd been 20 years older ) i never wore maternity clothes (thereby saving loads of money) and just bought size 10 stuff from primark, I was up and going for a jog while dd slept a week after having her!
I think many of my friends and contempories would be brilliant mothers despite their young age but I appreciate most of them would probably have an abortion as they have long term career aspirations which would be made harder by having child.
And finally not all the tracksuit wearing girls with hoop earrings and croydon facelifts are bad mothers, I go to a young mums mother and baby group, I'm the only mum there with 5A*-Cs at GCSE, I'm the only one who went to a grammar school, I'm the only one there who likes to wear Jack Wills, Abercrombie and All Saints but that doesn't make me a better mother, I have a friend who's 16, she smoked from the age of 11 but quit when she got pregnant, she doesn't drink, she breast-feed for 9months, she buys fresh fruit and veg and makes healthy meals for her DD, they go and feed the ducks, walk in the local woods and read stories at bedtime. If you saw her, I have no doubt you would judge her in her kappa tracksuit with her pram. But she is as good a mother as any.
Gosh this is a bit of an epic post, sorry