Complex sentence: Stevens, who was only 22 when she joined the cast (she had recently finished playing the title role on radio soap "Kitty Foyle"), played the part from 1944 to the show's cancellation in 1960.
Split into simpler sentences: Stevens, who had recently finished playing the title role on the radio soap "Kitty Foyle", was only 22 when she joined the cast. She continued in the role from 1944 to the show's cancellation in 1960.