These six idols were trainees at their respective agencies for at least ten years before debuting their groups.
Idols undergo training as trainees at their companies for an undetermined amount of time before debuting. Some train for a year, some even train for fifteen years before debuting.
The following six idols underwent a rigorous training period as trainees for at least ten years to finally debut.
TWICEs Jihyo trained for about ten years before debuting with TWICE and she even trained alongside Suzy during her training period. Since she started training at JYP Entertainment at such an early age, she debuted at the fresh age of 19.