Originally Posted by USAC Rulebook - Road

1A16. A rider's racing age shall be his or her age on
December 31 of the current year. All references to age of
riders, race age groups, or age class shall be interpreted as
referring to racing age. The following terms refer to specific
age groups.
Racing Age ...Age Group
Under 10 ...Youth
10-18 ...Junior
19-22 ...Under 23
23-29 ...Elite
30+ ...Master
However, a rider’s cyclocross racing age shall be his or her
racing age on the first of January that is included in the same
cyclocross season.