Here’s a little more context for the quote in the title, with the key part in bold:
You know one of the realities we’re facing right now is that people are looking at the number of young people in church and saying where are they? Well, about half of them never were. In other words, the evangelical birthrate in the year 2010 is about half of what it was in 1960. So there are about half as many teenagers with the same number of couples.
(Be aware that the conversation is focused on the societal ramifications of aging populations and not on the subject of the quote, which is birthrates. The quote comes in the closing comments by Mohler.)