Just found this thread when looking for something else, so thought I'd add my two cents.
So I've just had my first two kids in the past 18 months.... I'm old AF for a new dad... had just turned 46 when my son was born in February 2021 and was 47 when my daughter was born a few months ago. Of course having a newborn and toddler at the same time is a challenge but I couldn't be happier. I've wanted to be a Dad forever, but life circumstances basically caused about a decade of delays for me.
I got married to my first wife in my late 20s and we were both perfectly happy to wait a bit... our original plan was to have kids in our early 30s. Well years went by and we kept putting it off. I sensed that she was starting to be hesitant about it but we pushed forward and started trying when I was maybe 33-34 and she was maybe 31-32.
Nothing happened for about a year to the point that I was starting to wonder if something was wrong then - boom - she got pregnant. Unfortunately, we had a miscarriage. This was when I was around 35. I didn't really know at the time, but that whole thing basically convinced her that she didn't want kids, but it took several more years and a lot of drama to get that out of her and we got divorced the year I turned 40.
Meanwhile, I had basically given up on the idea of having kids. I figured I would meet someone and get married again (which I did), but I also figured the person who I met would very likely be divorced and have kids and so I made myself fine with that... I really just wanted to have any kind of family I could get.
So fast-forward a few years and I met my future wife. She was divorced and a good bit younger than me (she was 32 when we met) and also wanted kids. So fast forward a few more years and we are married with two amazing kids.
The TL;DR version is if you think you want to have kids, just do it. I don't think anyone can ever be truely prepared.
Also - even if you're old, go for it, man. Obviously for women there are increasing risks as you get older, but still. Are there things that will be more challenging because I'm an old-ass dad? Sure there will. But nothing in life is guaranteed. If you want kids, make it happen because we only live once. Even only being a Dad for just under 2 years now, I can't imagine missing out on this stuff. I feel pretty lucky, but then this is something that I've wanted for a long time.
In terms of the upside of being an old-ass Dad, I do feel like I'm a more patient than I would have been in my 30s or certainly my 20s. And as others have talked about... I don't feel like I "missed out" on anything as I did a shit-ton of partying, traveling, etc., in my 20s and 30s.