I had mine at 24, but that was partly because I was already on a slightly unusual trajectory in terms of the whole uni/career thing. I'd been working full time since 17, and had gone to uni part time (still working full time) at 21. So in a sense I interrupted myself in an equivalent way to having a baby at 19/20, as some on this thread are suggesting is more practical.
For me, it worked out beautifully in that practical sense. Going back to uni (part time in my final undergrad year, followed by full time MSc) was a really nice way of building up to being an actual working mother which I am now. DS came along for the ride, basically - I sound glib and of course him being alongside me made things harder in lots of ways, but it was also really nice.
Hard to compare a first DC at 24 with a first DC say 5 or 10 years later, because obviously nobody gets to be in both positions to compare! I am sure there's something to it when people suggest you've more energy as a younger mother, and of course the odds are much more in your favour with getting and staying pregnant. Otoh, I think it's also true that you gain greater emotional maturity as you age, and that's a really beneficial thing for motherhood. I would have described myself as comparatively mature at 24, and so would the people around me, but actually I think I really had to learn on my feet to find the necessary patience and perspective and strength for mothering - which is undoubtedly true of everyone but I really believe it's also something that comes with age and so is much harder when you're younger.
A lot of people on MN often talk about the importance of 'enjoying' your child-free years. I only half get this tbh. Before I had DS, my master plan was that he'd be an only and so I'd be 'free' again pretty much once I hit my forties. I knew loads of women in their forties who were out and about and having fun and dating etc so that didn't seem like a bad deal at all, especially when I felt so ready for a child. I've since decided I'm not done having babies, which changes that a bit! But my perspective has shifted on that and actually I'm pretty content accepting that I'll have dependent children until I'm closer to 50. I like having a child. I have found it extremely compatible with doing all sorts of other things, even as a lone parent with limited practical family support. It's not an inconvenience I wish to minimise or delay or whatever.
At 24 I was earning a modest salary and not feeling especially confident that that would change. Post-graduation, that's changed a lot, and I do sometimes wonder if I might have waited and done other things first, if I'd anticipated it? Childless colleagues of comparable age and salary go on some pretty amazing holidays etc, and occasionally I think ooh, that would be nice... Then I go home to my son, or better yet I go away with him to admittedly a less outright adventurous destination, and I think - actually, this is definitely, definitely what I'd choose.
Long and rambly! I'm glad I had mine when I did. Tbh I think once you decide you're fundamentally ready for a child, you make it work and you bend the narrative to suit your circumstances. 22-24 is a funny one in that to some people it looks very young (e.g. middle class London families where offspring live at home years past that!) and to others, where having babies in the late teens is pretty normal, it's relatively old. I've grown from a very working class background into a very middle class one and I notice this in the different perspectives acquaintances have - a lot of work people express surprise when they realise I have a 5yo, old school friends never think it's odd.