Here is a link to the NY Times write up on Population Day and a scientific/mathematical look at this issue.
Some say that it matters, others say it doesn't matter. One reason it doesn't matter is that the more people there are, the greater the possibility of having another Einstein who will help find a way out to the mess. Static's show that when people's life expectancy and quality of life improves, they have smaller families. Helping the poorest of the poor will actually help our planet because when their quality of life improves and they are educated, they begin to resemble more affluent countries which decreases and stabilizes the population. When people live longer and are more affluent, they have smaller families. An aging population is actually good for the planet.