Look, if scientists and researchers had more than a rudimentary understanding of how things work, they would have been able to double and triple our lifespan long ago.
Let's hope not. We are overpopulated as it is.
Here's a website that says that the land area of the United States is 9,181,859.76 -
http://www.worldatlas.com/aatlas/populations/usaareal.htm.
If there were 330,000,000 people in the U.S., and they were split into families of 4 on average, and each family lived in an apartment of 2,000 square feet, there would be 165,000,000,000 square feet of living space used by all 330,000,000 people.
There are 27878400 square feet in a square mile.
Divide the total square feet of living space by the number of square feet in a square mile. The result is slightly less than 6,000 square miles. This is the amount of land that living space would cover.
Stack the apartments in complexes 4 levels high to make room for roads and such between the apartments. This would leave 4,500 of the 6,000 square miles of residence land area for roads and parks and ponds and gardens and whatever.
The point is, even though living conditions might in some way be cramped in the above scenario, they wouldn't be SO cramped that people could not live. There would be over 9 million square miles of land left for everything else - from farming to manufacturing to whatever.
Over 99% of the idea that we are overpopulated is BS propaganda, designed to herd the people into the cities where they can be controlled by big government.
While there are all kinds of other considerations one might take into account, realize that the overpopulation propaganda is not giving you near enough of the true facts and figures.