Surely you have a cut off on the lower bound below which you don't offer this advice. 21? 18? 16? 13?
Yes, it's possible to put it off long enough thinking you'll find that perfect time, and either it getting to be too late, or perhaps running into the reality that with age comes higher likelihood of birth defects, in many cases. But the flat advice to just do it seems like it's missing a bit of at least a basic foundation advised.
I worked with folks who had kids in high school. They made it work, and had a different cultural background, but nothing about it seemed like the best idea -- at LEAST they had multigenerational households to cover some of the otherwise likely insurmountable difficulties particularly on the budgets they had working service industry jobs.
On your last paragraph that's how human raised kids for the first 10k+ years. In small groups of multigenerational settings.
Only in the last 100 (maybe even less, say 60) or so years we've started to say fk that, when I retire I want to be on a beach and not taking care of my grandkids.
It seems like the experiment of retirement is a recent phenomenon and one that the jury is still out in terms of whether it's a good idea, let alone sustainable.
Before 100 years ago there was precious little nation-state level social security, so families were far more dependent upon other family members. Middle age parents needed to support their parents directly, so there was an obvious quid-pro-quo for the grandparent to take care of the child.
Our forefathers changed this dynamic by making this arrangement somewhat invisible, in that we pay social security tax instead. We still take care of our parents, but not directly, and it's by the gun of the tax man rather than by a family arrangement. There is little incentive for this kind of unspoken bargaining of the past where you may take care of shelter and the grandparents take care of a kid. There's an IRS agent waiting with a gun if you don't pay social security, so any unspoken bargaining power is gone and the working age parent basically gets the squeeze from both sides.
While this might be right for overall humanity, for white collar professional workers specifically (typical audience of HN), I think late 20's / early 30's is a pretty good time to have kids - people have a solid 10 years of career experience under their belt, proportional amount of savings which help with both a financial cushion and with down payment for a house and some emotional maturity. On the other hand, one can get all sorts of experiences in 20's and their novelty starts wearing off. Finally, one is still young enough to have enough energy for child rearing.
A guy I worked with told me this advice and I wish I listened. We still had a kid early relatively to my other friends, I was 29 and wife was 27. It's better to be chasing around young kids when your 24 vs 34 I'll tell you that. But of course you're only as old as you feel.
I had my first at 32, and that was earlier than my friends. And when I pick mine up at daycare, I feel like I am the youngest. But I also do not feel noticeably worse off physically in mid 30s compared to mid 20s.
I disagree that there will never be a better situation. A couple with secure cash flows and in demand talents can optimize when they have kids to accommodate goals (for example, their preferred living situation, career goals, etc).
Things like moving, and working strenuous roles while having kids seems less than ideal. Of course, if you want 5 kids then waiting is not an option, but if you want 2, then there is some room for maneuvering.
Of course there are better situations, there's just no perfect situation. Your advice entails people should have kids in high school with their sweetheart if they want to. That's obviously wrong.
At worst their advice is just as wrong as the conventional wisdom they're contradicting.
I would say that the major factor isn't age, it's situation. If you are in a stable relationship with someone who can help financially and emotionally support an additional human, having a child could be an option.
If you are in a precarious situation, do not bring another human into it.
Pretty consistently research has found waiting until at least roughly 30 or so leads to greater average life satisfaction for parents. I'm not going to provide a rigorous or even cited defense here but I spent several days sifting through the studies before coming to this conclusion. I encourage the reader to also study the journals before coming to their own conclusion.
If you want children, do not wait. It will never be 'better situation'. There will be always some reason, mostly financial not to have them.