Here's a link to WERA's online rulebook (
WERA Motorcyle Road Racing :: Rulebook :: v2008n3x30 :: )
Per WERA's online rulebook in Chapter 7 concerning rider age requirements:
5. Any rider under the age of 18 may not compete without the duly notarized consent of parents or legal guardian. The notarized consent will be retained by WERA. In addition, each rider under the age of 18 must have on file with WERA a minor release form which can be obtained from WERA. Some facilities may require this form every weekend, the forms will be available at registration. Rider 8 years old may ride in practice onlywith the mini's. Riders 9 years old are limited to 50cc & 80cc classes. Riders 10 years old are restricted to the 125cc GP class. Riders 12 & 13 years old are restricted to 650cc LW Twins machines. Riders 14 & 15 years old are limited to “C” class machinery. Riders 16 and older are allowed to run any WERA legal machinery. These limits may be changed at WERA’s discretion on a case by case basis.