The following is a description of the rules regarding the types of gears allowed for junior racers in USA Cycling events. Most athletes with adult bikes (and some kids road bike) will need to have their hardest 1-2 gears “locked out”, meaning that the derailleur will not move the chain into those gears. This is a simple process that AERO coaches can show parents how to do. Please note that these restrictions apply only to USA Cycling events (Criteriums and Time trial bike races) and not to triathlons. Athletes participating in weekly crit races will have to decide whether to leave their hardest gears locked out or to changes them back and forth between triathlons and bike races.
These rules currently apply only to USA cycling events such as time trials and criterium races. While there are currently no restrictions for junior gearing in triathlon, we recommend that athletes remain locked out continuously.
From USA Cycling Website:
Junior Gear Restrictions: Is Your Race Bike legal?
Junior gearing can be a surprise to young athletes trying the sport of cycling for the first time. The young athlete attending his first USA Cycling-sanctioned event may be shocked to find his bicycle considered illegal. Parents may also wonder why this is so, especially when they have paid a lot of money for a bicycle they were told was race-ready. The main purpose of junior gear restrictions is to help the young rider develop a good pedal cadence and to avoid injury. Junior gear restrictions also level the playing field for developing juniors who may be at a disadvantage against rivals who possess physical advantages such as height and power.
The test to see if a race bike is legal or not is called the “rollout method” or simply “junior rollout”, which is the distance a bike travels backward in a straight line through one full pedal revolution when the bicycle is in its largest gear. The junior gear restriction for road events is 26 feet (7.93 meters).
To calculate how far a bike will travel relative to its gears, USA Cycling determines gear ratios with respect to race age and discipline and applies in all events in that discipline. There is no restriction for cyclo-cross or mountain bike races. For road and track the limits are*:
Ages 6-18: 7.93 meters (26’)(52x14)