Complex sentence: He was only 15-years-old when he joined BTS, being cast by seven agencies after auditioning for "Superstar K", but eventually chose Big Hit, reportedly after watching fellow member RM rap.
Split into simpler sentences: He was only 15-years-old when he joined BTS, being cast by seven agencies after auditioning for "Superstar K". He eventually chose Big Hit, reportedly after watching fellow member RM rap.