Marilyn Monroe was 19 in The Shocking Miss Pilgrim when she played the character 'Telephone Operator (voice) (uncredited)'.
That was over 77 years ago in 1947.
This was her first recorded movie acting credit (although may not technically be her first actual acting role).
In 1962 she died aged just 36, starring in 94 films in her career, 93 of those after The Shocking Miss Pilgrim was first released. If she were alive today she would be 97.
In The Shocking Miss Pilgrim, I think Marilyn Monroe looks: