Complex sentence: With the advent of World War I, first-class football was suspended, but despite his age (33), Sands continued to play for Arsenal in unofficial wartime matches, but later left to serve the Royal Army Medical Corps on the Western Front.
Split into simpler sentences: With the advent of World War I, first-class football was suspended, but despite his age (33), Sands continued to play for Arsenal in unofficial wartime matches. He later left to serve in the Royal Army Medical Corps on the Western Front.