Having a Scandinavian perspective on this, where birthrates also have fallen a lot recently, I don't think financial stability is the only factor. In my experience ppl here don't wait with having kids so long due to lack of money. With the social safety net we have here, free healthcare and education, paid parental leave etc, you are fine as long as you have a decent job. The reason that ppl wait until they are 35+ to get kids is rather that they want to do other shit first and just stay "young" longer and don't want the responsibility. Having kids is just not that important for ppl so they wait until they are kind of bored of other things and then they are often suddenly too old, or fail to find a good partner, or just don't want the responsibility.