Well, the topic sounds interesting to me. I would say before 25 for woman and before 30 for man.
The reason of this is, if you are too late having baby it will affect to your children's life and yours. For example, if you just get your first baby at the age of 35 years then when your child finished his/her university which is about 23 year old, you are already 58 years old at that time. You may need to work at least until that age before you retired.
And if you're not rich enough then you'll put the burden to your child. After completed his/her university, he needs to work very hard to support the family and may not able to continue his/her study.
This is just a thought, many of you probably won't agree with me.........
Everybody will disagree with me!
I say: 19.
Socially not accepted may-be but it's biologically the age that women are the most furtile and the womb is the freshest. Nowadays girls wait until they have financial certainties. It's socially the best choice perhaps but not biologically. In my country women get pregnant at an older age every year, and the number of children with down syndrome, increases with the same speed.