Giving birth to future tax payers should confer sizable tax deductions for the parents.

I'm not sure that's enough to reverse the demographic slide though, it's been tried.

For our ancestors, they married young, and didn't have access to birth control. Babies weren't really planned, they just happened.

they didn't just happen, they were expected and demanded. there was social pressure to have children. that's still true in china today. some not yet grandparents put a lot of pressure on their children to give them grandchildren (sometimes very violently too), and i remember a comment in an earlier thread where someone told about the experience of their parents or grandparents where the local pastor was having a concerned talk with a childless couple.