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.
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.
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.