Here is the rule as stated in the 2009 USAC Rule Book:

3.5 Age - Before a person shall be eligible for a license in the United States Auto Club (USAC), and participation in a USAC
sanctioned race meeting, such person must be eighteen (18) years of age or older. A driver under the age of eighteen (18)
years, but not less than sixteen (16) years of age, who has complied with Indiana Senate Enrolled Act No. 623, or like
recognized ACTS of other States enabling partial emancipation of a minor, will be eligible for consideration by USAC for an
annual membership/license(s) to compete in USAC National events.

On Selected oval paved tracks larger than 7/10 (0.70) mile all drivers must be eighteen (18) years of age. The partial
emancipation process of a minor will apply for events minors are allowed to participate and not apply for events minors
are not allowed to participate.

The entire rule is listed on page 4 of the USAC RULE BOOK.