Complex sentence: He was only 17 years old when he debuted in the 1996--97 season for the Patras-based club Panachaiki, he spent six seasons there and, by the third, was a regular starter.
Split into simpler sentences: He was only 17 years old when he debuted in the 1996--97 season for the Patras-based club Panachaiki. He spent six seasons there and, by the third, was a regular starter.