I'm not against that idea, but I think that came from thinking we "peak at 18" like I've heard my whole life.
I'm 54, a life-long athlete, and I'm in by far the best shape of my life, endless energy, the whole thing. If I was gonna put a number on it, I'd say about 30 - 45 is the right age to have kids.
Develop yourself mentally, spiritually, physically, and financially. Build a huge skillset. Learn how the world works. Then have kids. I think that's the ideal, though again, I'm not against a 20-year-old having kids at all. If that's your passion then go for it!
I just don't want people to think it's too late at 30 or 40 (or more!) It's not.