With a top heavy population pyramid, more resources have to be diverted to caring for the elderly, therefore less for raising the young - a self reinforcing mechanism.

When actually given the choice, many women are choosing not to have children, for good reasons. E.g. disruption of careers, physical effects on the body, being tied to a partner, financial drain.

Until we face up to both these issues, demographic collapse will continue - humanity will simply evaporate.