Schaefer, who was being treated at Albert Einstein Medical Center, was walking in the 5000 block of North Fifth Street on his way to a bus stop to meet his wife when the assailants pounced.
Surveillance video captured the teens walking a short distance behind Schaefer when they made a spur-of-the-moment decision to strike, said Philadelphia Police Lt. George McClay of the Northwest Detective Division.
"It's not even planned. They're walking down the street, laughing," McClay said. "One kid breaks off from the pack, beats him down, and the rest of them join in."
The assault was so severe it could have proved fatal, and police intend to charge the attackers with attempted murder, McClay said.
"Someone knows these kids," McClay said Thursday morning at a news conference in Olney. "They're from the neighborhood. We know they are. A teacher out there has seen them at school. We're looking for one or two names . . .. This behavior can't be let go. These animals have to be put away."
The assault prompted Fraternal Order of Police Lodge 5 to post a $1,000 reward for information leading to the arrest of the culprits.
The attackers were described as 16 to 18 years old, black and Hispanic. One was wearing an orange hooded sweatshirt and jeans.
Schaefer was shot in an eye in Vietnam, McClay said. Since then, he has worked in veterans' support services, helping others find jobs.
He has no recollection of the attack, McClay said.
"Last thing he knows, he's walking to the bus," he said.
Anyone with information is asked to call police at 215-686-3477.
Contact staff writer Allison Steele at 215-854-2641 or firstname.lastname@example.org.