found it!
http://www.usacycling.org/news/user/story.php?id=2706
1J6. Junior Gears. The maximum chaingear ratio for junior riders is based on age and discipline. Blocked gears will be allowed, except in National Championships. All tests for compliance (road and track) shall be done using the "roll-out method."
There is no restriction for cyclo-cross.
For road and track the limits are:
Road
17-18: 7.93 meters (26’) (52x14)
15-16: “
13-14: “
10-12: “
Track
17-18: Unrestricted
15-16: 6.78 meters (22'3") (48x15)
13-14: 6.36 meters (20'10.5") (48x16)
10-12: 6.00 meters (19’8”) (48x17)
for what its worth, 48x15 is what i typically ride on the track.