What is the current number of female PC gamers in the US, broken down by age?

I cannot find any figures broken down as you wish, but this should
give a general idea of the scale of women gamers:

# Forty-three percent of game players are women.
# Women over the age of 18 represent a greater portion of the
game-playing population than boys from 6 to 17 (28% and 21%
respectively).

(Source: http://www.theesa.com/facts/gamer_data.php)

Also, you may want to take a look at http://www.womengamers.com/, a
community that supports women gamers and trys to change how they are
overlooked etc; further information would definitely be able to be
found there.