If you want to spend time with your kids as adults, and have any grandkids remember you, younger is better. I was 30 when I had my first child; my parents might have been a little older than that when they had me. My dad died 20 years ago, my oldest son barely remembers him. My younger children technically met him but hav no memories of him. My mother died over 10 years ago.

My wife's mother had her when she was 20. She's still around, got to see her grandchildren reach adulthood, and have a long relationship with her daughter as an adult.