Seriously, isn't this planet overcrowded enough?
Funny enough, the problem is now that the population is dropping off too fast, like an inverted pyramid.
Population is stable at roughly a birthrate of 2.0 (per woman, over a life time). Birth rates in many countries are more like 1.4, even 1.1; I think I even saw something about China being at 0.9 or something, but I am less certain about that.
Now, there is a lag effect, but we're starting to hit the first stages of it. The elderly out number the young, and the ratio is only going to get worse. Part of what makes this a problem is that the elderly will need support in many aspects of their lives, and there are not enough young people to both provide the support the aging population will need and to also do all the other things that a society and economy needs.
Total population won't start to drop until a decade or two after this mismatch hits its peak and the largest age group starts dying off. Assuming there isn't major economic collapse before then.