The shooting was reported about 3:15 p.m. in the 4400 block of Turquoise Street, said LAPD Officer Aareon Jefferson, a spokesman for the department.
Police said a gun was found at the scene but have not said if the man was holding or reached for the weapon.
It's unclear what led to the shooting.
The man was taken to a hospital, and his condition was not available.
No officers were injured in the shooting, Jefferson said.
A multi-agency investigation was underway late Thursday, which is standard procedure when an LAPD officer opens fire.
The incident will be reviewed by the Los Angeles County district attorney's office, the Police Commission and its inspector general.
8:50 p.m.: This article was updated with the age of the man shot by police and the recovery of the weapon, and to note that the neighborhood is Montecito Heights, not El Sereno as authorities said previously.