In an earlier post on this topic ("Why definiteness is decreasing, part 1"), I suggested that the decrease in definite-article frequency in published English text, over the course of the past century, might be connected with a decrease in formality. Roughly, this means that writing has been becoming more like speech (though speech has also been changing, and writing and speech remain very different).
In this post, I want to discuss two other socio-stylistic dimensions — age and sex. If the language is changing, then we expect to see "age grading", where younger people tend to exhibit the innovative pattern, while older people's usage is more old-fashioned. And because women are generally the leaders in language change, we expect to see women at every age being more linguistically innovative and men being more conservative. In other words, "young men talk like old women". And as the plot on the right illustrates, differences by age and sex in the frequency of the seem to confirm this hypothesis. (Click on the graph for a larger version.)