The new 2006 rule book is out. The following is directly from the book.
1J6. Junior Gears. For Junior Men & Women, the authorized maximum chaingear ratio allowed in any road event (except cyclo-cross) is 7.93 meters (rollout 26'). [45x12 or 52x14 chain ring teeth]. Blocked gears will be allowed, except in National Championships. For track events the following limits shall be used: (10-12 years old- 6.00 meters or 19'8" [48x17], 13-14 years old- 6.36 meters or 20'10.5" [48x16], 15-16 yrs- 6.78 meters or 22'3" [48x15], 17+ yrs.- unrestricted.) All tests for compliance (road and track) shall be done using the "roll-out method."