Ok, I'm not a historian, but I am a physicist, so I will answer your question using a mathematical approach.

Let's say we want to compare the number of people living in 1900 with the number of people that had lived before 1900 but were strictly dead before 1900. Here, I will provide a very crude model (and I must stress, a VERY crude model) where we assume that the average life expectancy was around 40, so that the number of people that had lived before 1900 could be calculated from the total population in 1860 + the total population in 1820 + the total population in 1780 + etc. Let us also assume that the quadridecennial (40-yearly) growth rate, which we will call g was entirely constant throughout history. We can therefore calculate the total population backwards from 1900 using a geometric series p_init(Σg^(-r)), where p_init is the total population in 1900 (which was about 1.5 billion), r is an integer and one sums r from 1 to infinity (see http://en.wikipedia.org/wiki/Geometric_series for more info about what a geometric series is). This summation can be approximated to ((p_init)(g))/(1-g). Hence we want to find g so that p_init = ((p_init)(g))/(1-g).

This gives g equal 0.5, in other words historically the population would have had to double consistently every 40 years before 1900 for there to be more people living before 1900 than in 1900. As the population of the Earth rose much, much more gradually than that for the vast majority of history before 1900, 1900 clearly cannot be the year we are looking for.

So in answer to your question, if we wanted to find a point in time at which there were more people living than had lived before that point, we would have to find a point in time at which the human population would have experienced very rapid growth from something like Adam and Eve. Aside from having to be able to draw the line between what is considered human and what is considered, say, Homo Erectus, there have been occasions, such as the supereruption of Toba volcano, when the population of the Earth was almost completely decimated.

tl;dr unless you believe in the literal word of Genesis, there has almost certainly never been a point in history at which there were more people in the world living than had died before. The only way something like that could have happened is if, instead of looking at the population of the world as a whole, one looks only at the population of a single island, or even somewhere like the Americas or Australia, very, very early on in its history of human settlement.