Basically, the USCF has reinstated junior gear requirements

Junior Gear Requirements
Roll outs before start of race at the line.  Roll out distance of 26 feet.
Riders must present spare wheels at roll outs and will not be allowed to
leave staging area once roll outs are complete.
 
Riders can choose to meet gear restrictions through (1) blocking gears out
or (2) use of smaller front chain ring (i.e. 45) or larger cogs in the back
(i.e. starting cog of 15).
 
Please check USCF rule book for specific gearing recommendations.