Maybe some people realized that they could wait a few years and still have kids later, and others didn't think about having kids purely though the lens of evolutionary biology.
nitpick about what you said: you don't wanna postpone that plan too much. At 40 you won't have the energy to keep up with the needs of the kid. You will hate your life and you'll make a ton of mistakes. I had them young, and my career and financial situation weren't affected at all.
Before political correctness hit this area, doctors' observations led to classifying any first pregnancy past 25yo as geriatric. Healthcare progresses in some areas, some (like DS) are rigid to socially defined trends.
Okay? My parents had me at 35 in the 90s, and I'm sure you've met plenty of people who had similarly aged parents. It's not like there's one perfect age for everyone to have kids, and my point is that if some people realize at 25 that waiting five years puts them in a better position to raise kids, what's the problem with that?
Biologically there is an age range for all. Various complication risks raise pretty much continuously from about 20, for both the mother and the child. Say Down Syndrome is 5x more likely at 35 than at 20.
Last century geriatric pregnancy was one term used for over 35s. It is no longer used, and 25 seems like it is social media bullshit. Apparently the term was created ~1960 for international usage because medical complications after that age were far more risky in the 50s.