Although there are many factors that contribute to the total population of the world as Scott Mandrake writes in his Hub, this is not really what will cause decline.
The plague and famine killed tons of people but it did not have a reverse effect on population.
We will see a decline in population when we max out the world's resources. When the earth can no longer support us with its natural resources, we will start to see the population slow and eventually reverse.
When that day is, is tough to say. Who knows...we may even come up with technology that counters the depletion of resources. As a matter of fact, it is almost safe to say that we will cure death within the next 50-100 years!