Skin elasticity and athleticism decreases with age, and that translates into worse tearing, and more difficult births, longer recovery, and more trouble keeping up with a baby after all that.
Then, on top of all that, your parents are entering their sunset years rather where you have to help them rather than having grandparents young enough to help you with your kids.
It's also linked to things like low birth weight. I sure wish my wife & I had started earlier.