This thread is discussing OEEs. Employees under age 21/1YOS can be tested separately from everyone else. When they are, the OEEs usually have no HCEs or they all have a uniform allocation, and if so, the benefits given to the OEEs won’t need cross-testing to pass testing so no gateway applies to them, period. But, if the OEEs are needed to help the non-OEEs (over age 21/1YOS group) to pass testing, then any minimums that apply to those over 21/1 also apply to those under 21/1, such as a gateway.
Component plan testing is different in that it is an option that can be applied to the plan in a way that is not cut off at age 21/1YOS to delineate the components. If any component is cross-tested, then the gateway applies to all the components. If none of those components needed to use anyone under 21/1 to pass, the OEEs, then those OEEs are still not required to get the gateway unless the OEE group itself was cross-tested (super rare).