Complex sentence: He started playing the sport at the young age of 4, first joining Eastern AA's youth system for a chance to become an international footballer, then joining the Hong Kong Barcelona Football academy and training with players born in 2000.
Split into simpler sentences: He started playing the sport at the young age of 4, joining Eastern's youth system for a chance to become an international footballer. Dai later joined the Hong Kong Barcelona Football academy and trained with players born in 2000.