Not better babies - I do think it's harder on the mom's body to have a first baby later (by later I mean 40, around the time I was having my last)

This is all true, and I don't mean to discount the actual effects on the mother during pregnancy. There's a lot of variance with other health statistics/demographics, too. My wife is convinced that her being tall made her pregnancies easier, based on the not very scientific observation of "I can't imagine this stuff taking up this much space if I were shorter."

I'm a pretty active parent of elementary school age children, in a mixed income public school. Through my volunteering and involvement, I am friends and acquaintances with dozens of other parents of different ages, education levels, household incomes, family wealth. I can assure you, the challenges of parenthood are different for old parents and young parents, and there are real tradeoffs.

Personally, I was much more concerned about being old parents, until I actually became one and hung out with other parents. I think ideally, it's worth taking the time to get your shit together first before having kids. To me that means meaningfully getting to know yourself to where you have confidence and security and emotional maturity, screening for and building a good relationship with your future coparent (not marrying your high school or even college sweetheart), and getting your career off the ground. So I think 20-somethings will live their lives with plenty of good reasons to defer having children until around 30, and that there's nothing wrong with planning for a first child after 30, last child by 40, etc. I know enough moms who had kids between 40-45 that it's not that big a deal there, either.

Pregnancy is a tiny part of overall parenthood, and I'm glad I was closer to 30 when I met my wife, and that we waited until she was between 30-35 when we had our kids, because our lives, and our kids' lives, are better for how we worked on things between the ages of 25-30 (including simply being in love and doing fun child-free things).