I think that rather than all of our problems being caused by overpopulation, as some posters are suggesting, the majority of our economic problems are caused by economic liberalization. I don't have an opinion on the general, centuries-long and worldwide progress of economic liberalization, but I think that many of our current problems are really just frictional issues as we have moved from local economies in feudal states and unorganized political entities to global markets in modern nation-states.
A huge number of issues that have been talked about here are caused much less by overpopulation of the planet as a whole but instead by globalization. The United States has had its manufacturing base decimated by the outsourcing of much manufacturing to the Third World based on the relative wage advantage that those countries had. Luckily, some businesses have moved beyond such one-dimensional CBA and are beginning to bring manufacturing out of relatively inefficient locations.
The next issue that causes problems that might be confused for overpopulation is the rampant inequality present in some societies. It's easy to say "goodness, we must have too many people on earth" when we see grinding poverty, but the real issue is that all over the globe, capital-intensive industries and an unending drive to make the largest profit possible has created a class of people that were unable to afford their own level of education for their children, who were then less able to be productive members of the economy and were unable to afford their parents' level for their children. This is known to some people as "the cycle of poverty".
In order to counteract these forces, there are certain regulations that must be imposed on any society or economy in order to act in a responsible manner. It's important to protect children from working at a young age so that they have time to get an education and it's important that there is a floor on how low wages can go, so that everyone can afford to live in a dignified manner.
The main reason I feel that overpopulation is not in fact present is that we use almost all of our resources inefficiently. We prefer fossil fuels to any other form of energy, and when fossil fuels begin to run low, we're concerned that rather than there being a fundamental issue with using fossil fuels, there are too many people on the planet. We have abundant cropland, but the majority of that land is being used inefficiently as a result of lacking infrastructure, education, or market forces. We have abundant water, but many of us choose to live in places where our water resources are scant and thus tax the water resources of other places where those resources are better suited. There is certainly enough economic activity to provide everyone on earth with a job, but yet capital is increasingly concentrated away from those who create economic activity, consumers and producers, and towards a small number who largely take that capital out of the economy without putting into the economy an equal or greater amount in return.
There is a relatively simple remedy to an aging workforce, and that is immigration. There are countries on earth with demographics shaped like a mayan pyramid and others shaped like an upended dirigible. anyone can see that it is only logical, not political or irrational, to say that the answer to a demographic crisis is to allow young families to move from countries with poorly developed infrastructure to countries with adequate infrastructure.
Overall, I don't see overpopulation as anything close to being a single issue, but rather that there are a couple dozen issues that can be and should be dealt with as our world population increases. We need to invest in maintaining our water resources, developing our long-term, sustainable energy resources, bringing globalized economic liberty in line with globalized civil liberties, such as democracy and freedom of movement, investing in education and development for those in poverty, and developing a built environment that can equitably support future population increases.
Addendum:
First, 40% over forty years is not as simple as 40%/40=1%, it is in fact the fortieth root of 1.4, subtracting one and multiplying by 100, which comes out to about a 0.85% growth rate per year, whereas 100% over sixty years is about 1.16%. I have no issue with the point, just a correction on the math.
Second, every additional citizen is a consumer, with or without a job, as gwynedd1 noted. Even with capital intensive industries, people are still employed based on the amount that that is needed to be produced, just frequently in smaller numbers than labor intensive industries. The boom in employment in technological industries goes to show that even with declines in employment in traditional industries such as manufacturing and agriculture, innovation creates jobs.
Thirdly, there seems to be a lot of concern over birthrates among uneducated versus educated people. I'd like to direct them to this article.
Let's Not Panic Over Women With More Education Having Fewer Kids