Complex sentence: He began his thirty-four-year racing career as an apprentice jockey at age fifteen and by 1840 he was the dominant rider in British racing, winning the Champion Jockey title thirteen years in a row.
Split into simpler sentences: He began his thirty-four-year racing career as an apprentice jockey at age fifteen. By 1840 he was dominating British racing, winning the Champion Jockey title thirteen years in a row.