Complex sentence: Rooney married his first wife, Ava Gardner, Hollywood starlet at the age of 21 (she was 19) in 1942, but the two were divorced in 1943, well before she became a star in her own right.
Split into simpler sentences: Rooney married his first wife, Ava Gardner, in 1942; he was 21 and she was 19. The two were divorced in 1943, well before she became a Hollywood starlet in her own right.