Click on "Match Rules" on the left hand side, then click on
January 2008 Edition USPSA Handgun Rulebook on the right hand side.
EDIT TO ADD: I decided to look because of my own curiosity. IDPA has a minimum age, but it appears USPSA does not. I ventured over to the Brian Enos forum (THE place to go for USPSA discussions) and found a few threads talking about shooters as young as 8 and 10 at matches. If you can safely operate a 9mm or better through a course of fire, you're old enough.
I don't believe i have seen anything specifically say there is an age requirement. I have seen some younger shooters shoot, and the key is safety. If they can safely handle a weapon and are capible of completing a course of fire i think they should be fine.
How old is the child you are trying to get shooting? My recommendation would be to contact the Match director of the clubs you are interested in shooting at and see what their thoughts are. They all may have different opinions.
What clubs are you looking at shooting at? Are you a USPSA shooter yourself?