If I could live my life over, I would never have gone to grad school. And I'd have probably had kids 10 years earlier than I did.
- more overlap of your lifetime with the kids lifetime (so, see more of your kids) - get the scary parts over earlier, and enjoy more of the rest - if female, easier time conceiving
Let's put some numbers on that. My first child was born when I was 35. I turn 48 this year, and will not have all my children in college until I'm nearly 60. I'm not looking forward to figuring out college when I'd like to retire.
Moving on, my family health history sucks - I do not have a male relative who has reached 60 without a heart attack and/or stroke. That includes 2 brothers that I know of. I'm working to take care of myself with diet+exercise, but already have hypertension. If my children have children as late as I did, there won't be a grandchild until I'm in my 70s. I have very few male relatives who survived to 70, and none arrived there with their minds intact. There is reason to believe that I could do better, but realistically there is a good chance that I won't.
I was aware of most of these facts in my 20s. They just didn't seem relevant to the life choices that I was making then. In retrospect, I wish I'd chosen differently. I won't push my children towards any particular life decisions, but I hope to give them tools to make choices that better reflect what they want in the long term than I did.