Complex sentence: As filming for the "Lassie" 1964 season approached, Jon Provost was a fourteen-year-old actor who did not look forward to playing Timmy Martin another three years; he chose not renew his contract.
Split into simpler sentences: As filming for the "Lassie" 1964 season approached, Jon Provost was fourteen-years-old with his contract up for a three-year renewal. The actor did not look forward to playing Timmy Martin until the age of seventeen, and did not renew his contract.