Rulebook http://www.usacycling.org/forms/USAC_Rulebook.pdf page 50-51 says:
1M6. 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 51
compliance shall be done using the "roll-out method.” There
is no restriction for cyclo-cross or MTB races. For road and
track the limits are:
17-18: 7.93 meters (26‟)(52x14)
So if you look at my gear ratios and rollout tables below, i've highlighted the closest you can get to 26ft in common chainring sizes.
I attached the spreadsheet too so you can edit however you like....