“Women are about half the population. 17-18 yr old girls is just 1/80th or so of that half (assuming 80 yr lifespan).”
Keeping in mind the population growth , 18 year olds would be way more than 1/80th of the population. (just ask the founders of the NASI project).
Also, keep in mind that the 0-16 year olds are very unlikely to posting in the CR (at least I certainly hope so!), and since there are less people in every age over 18 (because of population growth), that means that the 18 years should represent for more than 1/80th of the people who could potentially be posting in the CR.
Furthermore, one could assume that wives and mothers are less likely to spend time in the CR as our Yeshiva bochurim/yungeleit. So it makes sense that 18 year old girls could represent a much greater percentage of those in the CR. They also probably spend much more time online than most others.
However, I would guess that despite their overrepresentation online, most of the topics in the CR don’t paricularly interest them (Trump,Clinton, etc), so they only post about those topics that interest them – namely seminary.