The current population crisis is as much of a technological problem as it is a sociological one. People simply live too long, but aren't able to remain productive during the late stage of their lives anymore. People live long, but aren't able to maintain quality of life for that long.
Old people weren't much of a problem a century ago because, well, being old meant being around your 40s-50s, an age in which the average human is still quite productive overall unless affected by serious injury or disease, and such things usualy resulted in death at that time anyway so it never became a (significant) social problem. Now we have this large chunk of the population with loads of expenses while being physically unable to be productive, in most cases.
However, it'd be nice if that was all the problem was about. Things are specially bad because the other half of the problem comes from a kind of social conditioning. In western (european, at least) society, people have been conditioned to just "enjoy life" and not worry about the problems the future generations will have to deal with. Even worse, a large chunk of the population decided that the best solution to that conundrum would to simply not have said next generation. There's a whole chunk of society who simply decided to not have any children because they thought there would be no real consequence to that, and that their personal pleasure ranks higher in the order of important things to do in life. Sure, in China that may not be a problem, but its a fuckhuge problem in yurop. Its not (purely) economical problem even, you can toss all the contraceptive methods and legalized abortion you want at, say, saudi arabia, and they'll still maintain a healthy population growth, simply because their culture encourages it.
Western europe has largely thrown away any semblance of its many cultures that stablish having children as a social duty. So, dear europa, stopstart fucking around.
There's also another economical problem that stems from people in developed countries being far more interested in manipulating money (investing) rather than actualy making money, but thats a whole 'nother can of worms.