She was 40 years old when she started filming I Love Lucy
Lucy was born on August 6, 1911. The pilot episode of I Love Lucy was shot sometime in September 1951. She was already in her late 30s when CBS approached her for a starring role in her own TV show, and she was 40 by the time the first episode was aired.
At the time, such a thing was practically unheard of in Hollywood. After 35, actresses were either pushed to the background or forced to play supporting roles, but not Lucy. In fact, she accomplished two major feats at that age: she got her huge break in Hollywood, and she became a mother for the first time just a month shy of turning 40.