Iseeall: You're right with regard to the best biological age to have at child...but when your nan was pregnant in the forties, society was very different. A lot of women didn't work and, even when they did, their husband was the main bread-winner, for a start. Also, people married younger, there were fewer educational prospects for women, and people died younger. Life was very different. You have a point, but sadly that's not really compatible with other things: how many people would go to university, travel the world, or have a good job at a young age if we all started families at 18-22? A lot fewer, I would imagine.
I'm 24 and will be 25 when my first arrives. I kinda feel 'old' and wish I'd done it a few years ago...but at the time, I could think of nothing worse. I'd have missed out a lot too as I've travelled quite extensively for someone from a working class family with f-all money, and I've had some interesting jobs that wouldn't have been possible with a family, and I've studied a lot. I also only got together with my partner just over a year ago, and there's never been anyone else I could imagine having a child with.
For people who've had children young - OP, perhaps you could answer? - how have you/how do you manage financially? The only ones I've known have either had an older partner whose already had a decent job, or have had substantial financial aid from their extended families. Most I've known/known of have been provided with somewhere to live by their families, had free childcare, been taken on free holidays so they don't miss out, and had a lot of money and support. I don't really see how it can be done otherwise - but I doubt my parents would have done it for me; I think they'd have been extremely disappointed and thrown me out/pushed me into an abortion (please note I'm not saying that's right by any means, and I'd certainly never do that to my daughter/son's gf if I',m put in that position).
Finally - sorry for going on and on - one of the best parents I know got pregnant at 17. She's the opposite of everything the media tries to tar teenage parents with, and she's done fantastically. I won't say too much for fear of outing myself, but she's a wonderful example of teenage parenting done right, and frankly I'm in awe of her and all she's achieved.