when did she start working in theatres?
November 12, 1928
Katharine Hepburn
Her Broadway debut came on November 12, 1928, at the Cort Theatre, but reviews for the show were poor, and it closed after eight nights. Hopkins promptly hired Hepburn as the lead understudy in Philip Barry's play "Holiday". 
was katharine hepburn an actress? Yes, an American actress.
wiki:11602152
Katharine Hepburn [SEP] Breaking into theatre (1928–1932)
In the spring of 1930, Hepburn joined a theatre company in Stockbridge, Massachusetts. She left halfway through the summer season, and continued studying with a drama tutor. In early 1931, she was cast in the Broadway production of "Art and Mrs. Bottle". She was released from the role after the playwright took a dislike to her, saying "She looks a fright, her manner is objectionable, and she has no talent", but Hepburn was re-hired when no other actress could be found. It went on to be a small success.