I've seen various stuff about this. A falling birth rate can mean that old people outnumber the young and make the economics of pensions difficult, but people are having less kids. Women are waiting longer to start a family. That seems to be a common trend as countries get better off. I don't think that is necessarily a bad thing as population cannot keep growing indefinitely. The world managed with less people before and in 50 years the balance of old and young may have levelled off.

I've heard that Africa will overtake Asia in population before things settle more.

What really matters is that everyone has a good chance of a decent life. If local population exhausts local water and food then you have issues. This is part of why people are migrating to Europe and the US. I think a lot of rubbish is talked about them 'replacing' us. A lot of that is just racism.

It's complicated and I'm no expert :)