Complex sentence: Thomas started his Winston Cup Career at the age of 35 and ended it when he was 48 years old; he was a competitor at least three major races of that era (the Fireball 300, the Tidewater 300, and the Yankee 400).
Split into simpler sentences: Thomas started his Winston Cup Career at the age of 35 and ended it when he was 48 years old. He was a competitor at least three major races of that era (the Fireball 300, the Tidewater 300, and the Yankee 400).