I agree it's an idealized timeline socially in a lot of media and things, though the research on it is more varying (though COVID may have an impact on previous findings), some of it supports that most make it, the quality is a bit more up for debate, but what's typical on a population level doesn't always matter on the individual one.
My brother married at 20, divorced 2 years later. I think he married because they'd been dating so long, but they were a young partying fun fit and not a life fit.
I married at 18, am still happily married nearly 20 years later. We've had our hard years, but I still view him and our marriage as the best choices I've ever made. I can imagine my life easier without our children than I can without him. My favourite way to start or end the day is just spending an hour or so talking and being with him. We never really partied together, but we're a life fit that has fun together.
Mostly pulling this out of my arse, but from my own observations, part of the issue for my brother and similar that I know is that their early relationships involved a lot of I guess stereotypical young people things - like I said, a good party fit getting drunk and having together which was good then, but not for life for most people. We grew up in an area with a lot of "you know it's for better or for worse" and "family gets through". There was little encouragement to consider what makes a good relationship or a good partner until things go wrong. You just relationship escalator it from one stage to the next and it either works or crashes and burns, often with a veneer period where things look fine while it sorts itself one way or the other or into grinding misery.
The books my parents got when I was in middle school and their relationship was pretty much undead with depressed teenagers, I was the weirdo kid reading those (I was also reading university brochures when I was 11, little-me was very future 'get me the fuck out of here' focused). I had a lot of teen-rocky relationships from typical teen-fuckery trying to figure things out, but it kinda clicked with my now-spouse - we talked through our views of children and careers before we even had sex (no idea why, did not plan that, just started talking and like now our conversation took a life of its own), we both had a medical emergency within a few months after we moved in together and had to figure out how to deal with that, our first extended argument (not a large one, it just took way longer than usual) was on trying to resolve our different religious perspectives -- and that's all changed, pretty much nothing then is how we live or think now and we're very different people now even medically, but we worked out how to work things out and that we can work things out with each other as we have similar values around the big life topics even if we come at it from different perspectives. We figured out how to be a life fit, though we have to refigure that out sometimes as we go through all the bumps in life.