Much of the world's population will decrease. We have been growing like mad in population, but this is just a spike in growth. Women used to spend 100% of their time caring for and bearing kids. A woman would give birth up to 8 times in her life and with luck 2 of these kids would survive. This changed with better sanitary conditions so at one point, many of these kids started to live on. You probably have grandparents from huge families, I do.

Now, bearing kids is expensive so people have less. There is a virtual guarantee that they'll survive. So a woman can give birth only twice. And make use of day care service. Thus women can have careers now, unlike 70 years ago. My great-grandmother was a housewife. Have sex has also gotten funner since it is not just a duty, but a pastime now. Imagine that? Young woman having hot sex by their own choice in a dorm since they are studying for a master's degree and/or a PhD. Traditonalists, freak the F!_!CK out!

So, people started to have less kids, the role of women changed, the family unit changed. No more getting married at 15 for women and at 21 for men. Some of this is good as it lets people make more choices, but some of it is bad as people like feminists and gays and others seek to progressively decouple the joint role that men and women perform in society.

The net result is, by having people become parents later (it is common to now marry at 28-32 or so unlike before) the average age of society, too, rises. Life expectancy has also risen making for an older population. It is all a change that we'll have to get used to.