That’s because young adulthood extends a bit longer now with a lot of people finishing uni at 21, further study, wanting to travel, building a career and then just wanting to enjoy adulthood and centring themselves.
Your DH obviously wanted to settle down with you and your kids- lovely.
but now people have so many options and there is little social pressure to be pushing a pram down the street by 25 less people think you are a spinster.
I had my first at 29 which was perfect for me. I had about a decade of living in a big city, travelling the world and building my career.
Once you have kids you need to negotiate and check with the other parent whether if it’s ok you go for an extra long bath- let alone going on a solo trip. People on mumsnet will be looking it from that angle which is why ‘under 25’ seems to be young. Most people at 25 are fancy free. Being a stepfather to 3 and one on the way at that age is rare.