Complex sentence: Yoon Ho, who is the maknae among the twelve trainees, is the member who has been the longest trainee in the company and he was meant to debut as a member of Boyfriend, but was too young to do so.
Split into simpler sentences: Yoon Ho, who is the maknae among the twelve trainees, is the member who has been the longest trainee in the company. He was meant to debut as a member of Boyfriend, but was too young to do so.