The victim had just exited a trolley about 8 p.m. June 13 when the assailant jumped him, punching him until he fell to the ground and then kicking him in the head, according to police.
By the time officers arrived, the attacker had fled, Lt. Angela DeSarro said. Police do not know why the victim was targeted.
The man, whose name was withheld, was taken to Alvarado Hospital, where he was treated for non-life-threatening injuries.
A surveillance camera captured images of the aggressor, described as a black teenager with braided hair, wearing blue jeans, a white sweat shirt, red tennis shoes and a black baseball cap.
Anyone with information about the case was asked to call San Diego County Crime Stoppers at (888) 580-8477 or contact the agency online at sdcrimestoppers.org. Tipsters may remain anonymous and could be eligible for a reward of up to $1,000.